Isolation and characterization of the ileal receptor for intrinsic factor-cobalamin

Vitamin B12 absorption and malabsorption

Vitamin B12 is assimilated and transported by complex mechanisms that involve three transport proteins, intrinsic factor (IF), haptocorrin (HC) and transcobalamin (TC) and their respective membrane receptors. Vitamin deficiency is mainly due to inadequate dietary intake in vegans, and B12 malabsorption is related to digestive diseases. This review explores the physiology of vitamin B12 absorption and the mechanisms and diseases that produce malabsorption. In the stomach, B12 is released from food carrier proteins and binds to HC. The degradation of HC by pancreatic proteases and the pH change trigger the transfer of B12 to IF in the duodenum. Cubilin and amnionless are the two components of the receptor that mediates the uptake of B12 in the distal ileum. Part of liver B12 is excreted in bile, and undergoes an enterohepatic circulation. The main causes of B12 malabsorption include inherited disorders (Intrinsic factor deficiency, Imerslund-Gräsbeck disease, Addison's pernicious anemia, obesity, bariatric surgery and gastrectomies. Other causes include pancreatic insufficiency, obstructive Jaundice, tropical sprue and celiac disease, bacterial overgrowth, parasitic infestations, Zollinger-Ellison syndrome, inflammatory bowel diseases, chronic radiation enteritis of the distal ileum and short bowel. The assessment of B12 deficit is recommended in the follow-up of subjects with bariatric surgery. The genetic causes of B12 malabsorption are probably underestimated in adult cases with B12 deficit. Despite its high prevalence in the general population and in the elderly, B12 malabsorption cannot be anymore assessed by the Schilling test, pointing out the urgent need for an equivalent reliable test.

Endolysosomal Disorders Affecting the Proximal Tubule of the Kidney: New Mechanistic Insights and Therapeutics

Epithelial cells that line the proximal tubule of the kidney rely on an intertwined ecosystem of vesicular membrane trafficking pathways to ensure the reabsorption of essential nutrients. To function effectively and to achieve homeostasis, these specialized cells require the sorting and recycling of a wide array of cell surface proteins within the endolysosomal network, including signaling receptors, nutrient transporters, ion channels, and polarity markers. The dysregulation of the endolysosomal system can lead to a generalized proximal tubule dysfunction, ultimately causing severe metabolic complications and kidney disease.In this chapter, we highlight the biological functions of the genes that code endolysosomal proteins from the perspective of understanding - and potentially reversing - the pathophysiology of endolysosomal disorders affecting the proximal tubule of the kidney. These insights might ultimately lead to potential treatments for currently intractable diseases and transform our ability to regulate kidney homeostasis and health.

Autoimmunity and Gastric Cancer

Alterations in the immune response of patients with autoimmune diseases may predispose to malignancies, and a link between chronic autoimmune gastritis and gastric cancer has been reported in many studies. Intestinal metaplasia with dysplasia of the gastric corpus-fundus mucosa and hyperplasia of chromaffin cells, which are typical features of late-stage autoimmune gastritis, are considered precursor lesions. Autoimmune gastritis has been associated with the development of two types of gastric neoplasms: intestinal type and type I gastric carcinoid. Here, we review the association of autoimmune gastritis with gastric cancer and other autoimmune features present in gastric neoplasms.

Receptor-Mediated Endocytosis in the Proximal Tubule

Cells lining the proximal tubule (PT) of the kidney are highly specialized for apical endocytosis of filtered proteins and small bioactive molecules from the glomerular ultrafiltrate to maintain essentially protein-free urine. Compromise of this pathway results in low molecular weight (LMW) proteinuria that can progress to end-stage kidney disease. This review describes our current understanding of the endocytic pathway and the multiligand receptors that mediate LMW protein uptake in PT cells, how these are regulated in response to physiologic cues, and the molecular basis of inherited diseases characterized by LMW proteinuria.

Dab2, megalin, cubilin and amnionless receptor complex might mediate intestinal endocytosis in the suckling rat

We previously proposed that Dab2 participates in the endocytosis of milk macromolecules in rat small intestine. Here we investigate the receptors that may mediate this endocytosis by studying the effects of age and diet on megalin, VLDLR, and ApoER2 expression, and that of age on the expression of cubilin and amnionless. Of megalin, VLDLR and ApoER2, only the megalin expression pattern resembles that of Dab2 previously reported. Thus the mRNA and protein levels of megalin and Dab2 are high in the intestine of the suckling rat, down-regulated by age and up-regulated by milk diet, mainly in the ileum. Neither age nor diet affect ApoER2 mRNA levels. The effect of age on VLDLR mRNA levels depends on the epithelial cell tested but they are down-regulated by milk diet. In the suckling rat, the intestinal expressions of both cubilin and amnionless are similar to that of megalin and megalin, cubilin, amnionless and Dab2 co-localize at the microvilli and in the apical endocytic apparatus. Co-localization of Dab2 with ApoER2 and VLDLR at the microvilli and in the apical endocytic apparatus is also observed. This is the first report showing intestinal co-localization of: megalin/cubilin/amnionless/Dab2, VLDLR/Dab2 and ApoER2/Dab2. We conclude that the megalin/cubilin/amnionless/Dab2 complex/es participate in intestinal processes, mainly during the lactation period and that Dab2 may act as an adaptor in intestinal processes mediated by ApoER2 and VLDLR.

Uptake and transport of B12-conjugated nanoparticles in airway epithelium

Non-invasive delivery of biotherapeutics, as an attractive alternative to injections, could potentially be achieved through the mucosal surfaces, utilizing nanoscale therapeutic carriers. However, nanoparticles do not readily cross the mucosal barriers, with the epithelium presenting a major barrier to their translocation. The transcytotic pathway of vitamin B12 has previously been shown to 'ferry' B12-decorated nanoparticles across intestinal epithelial (Caco-2) cells. However, such studies have not been reported for the airway epithelium. Furthermore, the presence in the airways of the cell machinery responsible for transepithelial trafficking of B12 is not widely reported. Using a combination of molecular biology and immunostaining techniques, our work demonstrates that the bronchial cell line, Calu-3, expresses the B12-intrinsic factor receptor, the transcobalamin II receptor and the transcobalamin II carrier protein. Importantly, the work showed that sub-200 nm model nanoparticles chemically conjugated to B12 were internalised and transported across the Calu-3 cell layers, with B12 conjugation not only enhancing cell uptake and transepithelial transport, but also influencing intracellular trafficking. Our work therefore demonstrates that the B12 endocytotic apparatus is not only present in this airway model, but also transports ligand-conjugated nanoparticles across polarised epithelial cells, indicating potential for B12-mediated delivery of nanoscale carriers of biotherapeutics across the airways.

Nanoparticle transport in epithelial cells: pathway switching through bioconjugation

The understanding and control of nanoparticle transport into and through cellular compartments is central to biomedical applications of nanotechnology. Here, it is shown that the transport pathway of 50 nm polystyrene nanoparticles decorated with vitamin B12 in epithelial cells is different compared to both soluble B12 ligand and unmodified nanoparticles, and this is not attributable to B12 recognition alone. Importantly, the study indicates that vitamin B12 -conjugated nanoparticles circumnavigate the lysosomal compartment, the destination of soluble vitamin B12 ligand. Whereas cellular trafficking of soluble B12 is confirmed to occur via the clathrin-mediated pathway, transport of B12 -conjugated nanoparticles appears to predominantly take place by a route that is perturbed by caveolae-specific inhibitors. This data suggests that, following its conjugation to nanoparticles, in addition to dramatically increasing the cellular uptake of nanoparticles, the normal cell trafficking of B12 is switched to an alternative pathway, omitting the lysosomal stage: a result with important implications for oral delivery of nanoparticulate diagnostics and therapeutics.

Gastric intrinsic factor: the gastric and small intestinal stages of cobalamin absorption. a personal journey

Intrinsic factor (IF) was first identified as a component of the gastric mucosa that reacted with an extrinsic factor, later discovered to be vitamin B12 (VB12). IF has been extensively characterized, and its cloned cDNA used to produce sufficient IF to produce high quality antibodies, and to elucidate its 3-dimensional structure bound to cobalamin (Cbl, VB12). The absorption of the IF-Cbl complex involves internalization by endocytosis, incorporation into multivesicular/lysosomal bodies, release of Cbl by lysosomal proteolysis and pH effects, with subsequent binding to transcobalamin (TC). Hereditary IF deficiency is rare, consistent with the need for IF to absorb Cbl, a vitamin essential for cell replication. When mutations occur, they are most often associated with loss of function, but some mutations occur outside the coding region. The IF-mediated intestinal uptake of Cbl has been harnessed for use as a transporter for peptides, proteins and even nanoparticles. Nanoparticle (NP) technology has produced Cbl-coated NPs that can incorporate peptides (insulin, IgG) that can be absorbed orally to function as hormones and antibodies in rodent models, but these systems are not yet ready for clinical use.

Investigations into the etiology of neural tube defects

Neural tube defects (NTDs) are serious malformations affecting approximately 1 per 1000 births, yet the mechanisms by which they arise are unknown. There have been consistent efforts in many fields of research to elucidate the etiology of this multifactorial condition. While no single gene has been identified as a major independent risk factor for NTDs, candidate genes have been proposed that may modify the effects of maternal and/or embryonic exposures. Folate supplementation effectively reduces the occurrence of NTDs and, consequently, has focused much research on metabolism of folate-related pathways during pregnancy and development. Further understanding of normal development and how teratogens can perturb these orchestrated processes also remains at the fore of modern scientific endeavors. The composite of these factors remains fragmented; the aim of this review is to provide the reader with a summary of sentinel and current works in the body of literature addressing NTD disease etiology.

Mouse amnionless, which is required for primitive streak assembly,mediates cell-surface localization and endocytic function of cubilin on visceral endoderm and kidney proximal tubules

Impaired primitive streak assembly in the mouse amnionless(amn) mutant results in the absence of non-axial trunk mesoderm, a derivative of the middle region of the primitive streak. In addition, the epiblast of amn mutants fails to increase significantly in size after E7.0, indicating that middle primitive streak assembly is mechanistically tied to the growth of the embryo during gastrulation. Amn, a novel transmembrane protein, is expressed exclusively in an extra-embryonic tissue, visceral endoderm (VE), during the early post-implantation stages. We show that Amn is also expressed in kidney proximal tubules (KPT) and intestinal epithelium,which, like the VE, are polarized epithelia specialized for resorption and secretion. To explore whether Amn participates in the development or function of KPT and intestinal epithelia and to gain insight into the function of Amn during gastrulation, we constructed Amn-/- ES cell↔+/+blastocyst chimeras. While chimeras form anatomically normal kidneys and intestine, they exhibit variable, selective proteinuria, a sign of KPT malfunction. In humans, AMN has been genetically connected to Cubilin(CUBN), a multi-ligand scavenger receptor expressed by KPT, intestine and yolk sac. Loss of CUBN, the intestinal intrinsic factor (IF)-vitamin B12 receptor, results in hereditary megaloblastic anemia (MGA1), owing to vitamin B12 malabsorption. The recent report of MGA1 families with mutations in AMN suggests that AMN functions in the same pathway as CUBN. We demonstrate that Cubn is not properly localized to the cell surface in Amn-/- tissues in the embryo and adult mouse, and that adult chimeras exhibit selective proteinuria of Cubn ligands. This study demonstrates that Amn is an essential component of the Cubn receptor complex in vivo and suggests that Amn/Cubn is required for endocytosis/transcytosis of one or more ligands in the VE during gastrulation to coordinate growth and patterning of the embryo. Furthermore, as AMN is apparently not required for gastrulation in humans, the developmental requirements for Amn/Cubn function may not be evolutionarily conserved, possibly reflecting differences between species in the role and organization of extra-embryonic tissues.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Mechanisms of vitamin B(12) absorption in breast-fed infants

OBJECTIVES

The mechanisms of vitamin B(12) absorption in infants are unknown. We investigated whether haptocorrin (HC), a vitamin B(12) -binding protein in human milk, facilitates vitamin B(12) absorption during the neonatal period or if it occurs by a process similar to that in adults involving another vitamin B(12) -binding protein, intrinsic factor (IF).

METHODS

To determine whether HC or IF can deliver vitamin B(12) to the enterocyte, binding studies using Caco-2 intestinal cells in culture and purified human milk HC-[ (57)Co]vitamin B(12) or [(125)I]IF-vitamin B(12) were performed. Determination of IF secretion by infant stomach was investigated by a competitive ELISA on fecal extracts from breast-fed infants. Determination of receptors specific for IF-vitamin B(12) or HC-vitamin B(12) in infant intestine was achieved by ligand blot analysis using isolated brush border membrane vesicles (BBMV) from fetal and adult intestine and Caco-2 cells. PCR was performed to identify the IF receptor gene transcript in Caco-2 cells and fetal intestine.

RESULTS

Limited binding of both HC and IF to Caco-2 cells was observed; however, HC displayed affinity to low molecular weight proteins in BBMV from fetal intestine and Caco-2 cells while IF showed affinity for a 240 kDa protein in BBMV from fetal intestine and Caco-2 cells. IF receptor gene transcript was identified in fetal intestine and Caco-2 cells. An increase in IF excretion from breast-fed infants throughout early life was observed.

CONCLUSIONS

An IF-dependent vitamin B(12) absorption mechanism appears to be in place in breast-fed infants. However, IF levels may be too low in early life to participate in vitamin B(12) absorption; therefore, haptocorrin may mediate vitamin B(12) absorption until the absorption function can be taken over by a more mature IF system.

Loss of albumin and megalin binding to renal cubilin in rats results in albuminuria after total body irradiation

The role of the renal apical brush-border membrane (BBM) endocytic receptors cubilin and megalin in the onset of albuminuria in rats exposed to a single dose of total body irradiation (TBI) has been investigated. Albuminuria was evident as immunoblot (IB) analysis of the urine samples from TBI rats revealed excretion of large amounts of albumin. IB analysis of the BBM proteins did not reveal any significant changes in cubilin or megalin levels, but (125)I-albumin binding to BBM from TBI rats declined by 80% with a fivefold decrease (from 0.5 to 2.5 microM) in the affinity for albumin. IB analysis of cubilin from the BBM demonstrated a 75% loss when purified using albumin, but not intrinsic factor (IF)-cobalamin (Cbl) ligand affinity chromatography. Immunoprecipitation (IP) of Triton X-100 extract of the BBM with antiserum to cubilin followed by IB of the immune complex with an antiserum to megalin revealed a 75% loss of association between megalin and cubilin. IP studies with antiserum to cubilin or megalin and IB with antiserum to the cation-independent mannose 6-phosphate/insulin-like growth factor II-receptor (CIMPR) revealed that CIMPR interacted with both cubilin and megalin. In addition, TBI did not disrupt the association of CIMPR with either cubilin or megalin in BBM. These results suggest that albuminuria noted in TBI rats is due to selective loss of albumin and megalin, but not CIMPR or IF-Cbl binding by cubilin. Furthermore, these results also suggest that albumin and IF-Cbl binding to cubilin occur at distinct sites and that in the rat renal BBM, CIMPR interacts with both cubilin and megalin.

Identification and characterization of two distinct ligand binding regions of cubilin

Using polymerase chain reaction-amplified fragments of cubilin, an endocytic receptor of molecular mass 460 kDa, we have identified two distinct ligand binding regions. Region 1 of molecular mass 71 kDa, which included the 113-residue N terminus along with the eight epidermal growth factor (EGF)-like repeats and CUB domains 1 and 2, and region 2 of molecular mass 37 kDa consisting of CUB domains 6-8 bound both intrinsic factor-cobalamin (vitamin B(12); Cbl) (IF-Cbl) and albumin. Within these two regions, the binding of both ligands was confined to a 110-115-residue stretch that encompassed either the 113-residue N terminus or CUB domain 7 and 8. Ca(2+) dependence of ligand binding or the ability of cubilin antiserum to inhibit ligand binding to the 113-residue N terminus was 60-65%. However, a combination of CUB domains 7 and 8 or 6-8 was needed to demonstrate significant Ca(2+) dependence or inhibition of ligand binding by cubilin antiserum. Antiserum to EGF inhibited albumin but not IF-Cbl binding to the N-terminal cubilin fragment that included the eight EGF-like repeats. While the presence of excess albumin had no effect on binding to IF-Cbl, IF-Cbl in excess was able to inhibit albumin binding to both regions of cubilin. Reductive alkylation of the 113-residue N terminus or CUB 6-8, CUB 7, or CUB 8 domain resulted in the abolishment of ligand binding. These results indicate that (a) cubilin contains two distinct regions that bind both IF-Cbl and albumin and that (b) binding of both IF-Cbl and albumin to each of these regions can be distinguished and is regulated by the nonassisted formation of local disulfide bonds.

Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule

The multiligand, endocytic receptors megalin and cubilin are colocalized in the renal proximal tubule. They are heavily expressed in the apical endocytic apparatus. Megalin is a 600-kDa transmembrane protein belonging to the low-density lipoprotein-receptor family. The cytoplasmic tail contains three NPXY motifs that mediate the clustering in coated pits and are possibly involved in signaling functions. Cubilin, also known as the intestinal intrinsic factor-cobalamin receptor, is a 460-kDa receptor with no transmembrane domain and no known signal for endocytosis. Because the two receptors bind each other with high affinity and colocalize in several tissues, it is highly conceivable that megalin mediates internalization of cubilin and its ligands. Both receptors are important for normal tubular reabsorption of proteins, including albumin. Among the proteins normally filtered in the glomeruli, cubilin has been shown to bind albumin, immunoglobulin light chains, and apolipoprotein A-I. The variety of filtered ligands identified for megalin include vitamin-binding proteins, hormones, enzymes, apolipoprotein H, albumin, and beta(2)- and alpha(1)-microglobulin. Loss of these proteins and vitamins in the urine of megalin-deficient mice illustrates the physiological importance of this receptor.

Cubilin expression and posttranslational modification in the canine gastrointestinal tract

Cubilin is an endocytic receptor of the apical brush border membrane that is essential for intrinsic factor-mediated cobalamin absorption in small intestine. However, cubilin is more highly expressed in kidney and yolk sac, and recent molecular characterization of the receptor has focused on these tissues. The aim of this investigation was to examine tissue-specific cubilin expression and posttranslational modifications with an emphasis on the gastrointestinal tract. Intrinsic factor-cobalamin binding activity, cubilin immunoreactivity, and cubilin mRNA levels were determined in multiple segments of canine gastrointestinal mucosa and other tissues. These aspects of cubilin expression varied in parallel, suggesting that the major determinant of regional cubilin expression in the gastrointestinal tract is modulation of cubilin mRNA. Cell fractionation indicated that ileal cubilin is not strongly membrane associated. An approximately 185-kDa brush border specific and two >400-kDa precursor forms of cubilin were identified. Asparagine-linked oligosaccharide modifications characterized by differential glycosidase digestion of affinity-purified cubilin from ileal mucosa and renal cortex differed, but ileal and renal intracellular cubilin comigrated on SDS-PAGE at approximately 400 kDa after oligosaccharide removal, thus reconciling previous conflicting size estimates of the cubilin polypeptide.

Transcobalamin II and its cell surface receptor

Transcobalamin II (TC II), a nonglycoprotein secretory protein of molecular mass 43 kDa, and its plasma membrane receptor (TC II-R), a heavily glycosylated protein with a monomeric molecular mass of 62 kDa, are essential components of plasma cobalamin (Cbl; vitamin B12) transport to all cells. Evidence from studies over the past 10 years has provided some important information on their structure, regulation of expression, and function. Some of the specific findings include (a) identification of the structural relationship of the ligand TC II with other members of the Cbl-binding family of proteins, intrinsic factor (IF) and haptocorrin (HC), (b) regulation of TC II gene expression, (c) molecular basis for human TC II deficiency in patients with a lack of plasma TC II, (d) membrane expression, interactions, and dimerization of TC II-R, and (e) targeting and function of TC II-R in polarized epithelial cells. It is hoped that some of the recent findings presented in this review will provide new insights into the structure and function of these two fascinating proteins and stimulate future research in this area.

Functional expression in Pichia pastoris of human and rat intrinsic factor

Intrinsic factor (IF) has been expressed previously in Baculovirus with a yield (0.1-1 mg/l) that was inadequate for structural and metabolic studies. IF cDNAs were cloned into the shuttle vector pPIC9 of P. pastoris, and the proteins were induced and purified by cobalamin (Cbl) affinity chromatography. Expression of recombinant proteins revealed a major band of 49 kDa for both human and rat IF. Expression of human IF was achieved at 1040 mg/l, but of rat IF at only 1-2 mg/l. Reaction of human IF with a photo-activatable derivative of Cbl was demonstrated by Western blotting, and detection of IF fragments by anti-Cbl monoclonal antibody and by amino-terminal sequencing revealed at least three regions (residues 129-151, 234-254, and +294) linked to Cbl. Both recombinant human and rat [125I]IF-Cbl bound to rat and guinea pig brush border membranes with similar affinity, but the binding capacity of human IF for the rat receptor was only 10% compared with rat IF. All six amino acids within the previously identified N-terminal binding region of human IF were mutated to be identical to rat IF, but the resulting chimeric IF still bound poorly to rat membranes. Mutations of residues 26/27 (Glu26 to Asp and Asn27 to Gln) and 32/34 (Ser32 to Thr and Tyr34 to Arg) showed changes in both Ka and Vmax, with great effects on Vmax. In conclusion, P. pastoris is an expression system that produces functional human IF at a higher yield than in the baculovirus system. Cbl binding was directly demonstrated at multiple sites along the linear sequence of human IF. The receptor binding function of the amino terminal sequence 25 62 has been confirmed, but it is insufficient to reproduce all the features of IF-Cbl binding.

Vitamin B12 deficiency in the elderly

Vitamin B12 deficiency is estimated to affect 10%-15% of people over the age of 60, and the laboratory diagnosis is usually based on low serum vitamin B12 levels or elevated serum methylmalonic acid and homocysteine levels. Although elderly people with low vitamin B12 status frequently lack the classical signs and symptoms of vitamin B12 deficiency, e.g. megaloblastic anemia, precise evaluation and treatment in this population is important. Absorption of crystalline vitamin B12 does not decline with advancing age. However, compared with the younger population, absorption of protein-bound vitamin B12 is decreased in the elderly, owing to a high prevalence of atrophic gastritis in this age group. Atrophic gastritis results in a low acid-pepsin secretion by the gastric mucosa, which in turn results in a reduced release of free vitamin B12 from food proteins. Furthermore, hypochlorhydria in atrophic gastritis results in bacterial overgrowth of the stomach and small intestine, and these bacteria may bind vitamin B12 for their own use. The ability to absorb crystalline vitamin B12 remains intact in older people with atrophic gastritis. The 1998 recommended daily allowance for vitamin B12 is 2.4 micrograms, but elderly people should try to obtain their vitamin B12 from either supplements or fortified foods (e.g. fortified ready-to-eat breakfast cereals) to ensure adequate absorption from the gastrointestinal tract. Because the American food supply is now being fortified with folic acid, concern is increasing about neurologic exacerbation in individuals with marginal vitamin B12 status and high-dose folate intake.

The intrinsic factor-vitamin B12 receptor, cubilin, is assembled into trimers via a coiled-coil alpha-helix

A large protein was purified from bovine kidney, using selective extraction with EDTA to solubilize proteins anchored by divalent cation-dependent interactions. An antiserum raised against the purified protein labeled the apical cell surface of the epithelial cells in proximal tubules and the luminal surface of small intestine. Ten peptide sequences, derived from the protein, all matched the recently published sequences for rat (Moestrup, S. K., Kozyraki, R., Kristiansen, M., Kaysen, J. H., Holm Rasmussen, H., Brault, D., Pontillon, F., Goda, F. O., Christensen, E. I., Hammond, T. G., and Verroust, P. J. (1998) J. Biol. Chem. 273, 5235-5242) and human cubilin, a receptor for intrinsic factor-vitamin B12 complexes, identifying the protein as bovine cubilin. In electron microscopy, a three-armed structure was seen, indicating an oligomerization of three identical subunits. This model was supported by the Mr values of about 1,500,000 for the intact protein and 440,000 for its subunits obtained by analytical ultracentrifugation. In a search for a potential assembly domain, we identified a region of heptad repeats in the N-terminal part of the cubilin sequence. Computer-assisted analysis supported the presence of a coiled-coil alpha-helix between amino acids 103 and 132 of the human cubilin sequence and predicted the formation of a triple coiled-coil. We therefore conclude that cubilin forms a noncovalent trimer of identical subunits connected by an N-terminal coiled-coil alpha-helix.

Overexpression of an unstable intrinsic factor-cobalamin receptor in Imerslund-Gräsbeck syndrome

Two sisters with Imerslund-Gräsbeck syndrome who presented with clinical features of cobalamin deficiency are described. Intrinsic factor-cobalamin receptor (IFCR) activity and protein levels were determined in ileal biopsy specimens by using radioisotope assay and immunoblotting, respectively. IFCR activities in ileal homogenates expressed as femtomoles of ligand binding per milligram of protein were 38 +/- 4 in control tissue, 494 +/- 24 in patient 1, and 94 +/- 7 in patient 2. However, when assayed in the presence of IFCR antiserum, the ligand binding was inhibited by > 90% in both normal control and the patients with Imerslund-Gräsbeck syndrome. Immunoblotting of total membranes from the biopsy specimen of these 2 patients failed to detect an immunoreactive band of molecular mass of 185 kilodaltons. These findings are at variance with reports of decreased IFCR activity and indicate a new phenotype in which an active but an unstable receptor is overexpressed in Imerslund-Gräsbeck syndrome.

The intrinsic factor-vitamin B12 receptor and target of teratogenic antibodies is a megalin-binding peripheral membrane protein with homology to developmental proteins

The present report shows the molecular characterization of the rat 460-kDa epithelial glycoprotein that functions as the receptor facilitating uptake of intrinsic factor-vitamin B12 complexes in the intestine and kidney. The same receptor represents also the yolk sac target for teratogenic antibodies causing fetal malformations in rats. Determination of its primary structure by cDNA cloning identified a novel type of peripheral membrane receptor characterized by a cluster of eight epidermal growth factor type domains followed by a cluster of 27 CUB domains. In accordance with the absence of a hydrophobic segment, the receptor could be released from renal cortex membranes by nonenzymatic and nonsolubilizing procedures. The primary structure has no similarity to known endocytic receptors but displays homology to epidermal growth factor and CUB domain proteins involved in fetal development, e.g. the bone morphogenic proteins. Electron microscopic immunogold double labeling of rat yolk sac and renal proximal tubules demonstrated subcellular colocalization with the endocytic receptor megalin, which is expressed in the same epithelia as the 460-kDa receptor. Furthermore, megalin affinity chromatography and surface plasmon resonance analysis revealed a calcium-dependent high affinity binding of the 460-kDa receptor to megalin, which thereby may mediate its vesicular trafficking. Due to the high number of CUB domains, accounting for 88% of the protein mass, we propose the name cubilin for the novel receptor.

Characterization of an epithelial approximately 460-kDa protein that facilitates endocytosis of intrinsic factor-vitamin B12 and binds receptor-associated protein

By using receptor-associated protein (RAP) as an affinity target, an intrinsic factor-vitamin B12 (IF-B12)-binding renal epithelial protein of approximately 460 kDa was copurified together with the transcobalamin-B12-binding 600-kDa receptor, megalin. IF-B12 affinity chromatography of renal cortex membrane from rabbit and man yielded the same approximately 460-kDa protein. Binding studies including surface plasmon resonance analyses of the protein demonstrated a calcium-dependent and high affinity binding of IF-B12 to a site distinct from the RAP binding site. The high affinity binding of IF-B12 was dependent on complex formation with vitamin B12. Light and electron microscope autoradiography of rat renal cortex cryosections incubated directly with IF-57Co-B12 and rat proximal tubules microinjected in vivo with the radioligand demonstrated binding of the ligand to endocytic invaginations of proximal tubule membranes followed by endocytosis and targeting of vitamin B12 to lysosomes. Polyclonal antibodies recognizing the approximately 460-kDa receptor inhibited the uptake. Immunohistochemistry of kidney and intestine showed colocalization of the IF-B12 receptor and megalin in both tissues. In conclusion, we have identified the epithelial IF-B12-binding receptor as a approximately 460-kDa RAP-binding protein facilitating endocytosis.

In vitro and in vivo inactivation of transcobalamin II receptor by its antiserum

Rabbits injected with pure human placental transcobalamin II-receptor (TC II-R) failed to thrive with no apparent tissue or organ damage, but a 2-fold elevation of the metabolites, homocysteine, methylmalonic acid, and the ligand, transcobalamin II, in their plasma. Exogenously added transcobalamin II-[57Co]cyanocobalamin bound very poorly (2-5%) to the affected rabbit liver, kidney, and intestinal total or intestinal basolateral membrane extracts relative to the binding by membrane extracts from normal rabbit tissues. The activity was restored to normal values following a wash of affected rabbit tissue membranes with pH 3 buffer containing 200 mM potassium thiocyanate. Immunoblot analysis of normal and affected rabbit kidney and liver total membranes revealed similar amounts of 124-kDa TC II-R dimer protein. The neutralized and dialyzed extract from the affected rabbit membranes inhibited the binding of the ligand to pure TC II-R and the harvested affected rabbit serum inhibited the uptake of TC II-[57Co]cobalamin (Cbl) from the basolateral side of human intestinal epithelial (Caco-2) cells and decreased the utilization of [57Co]Cbl as coenzymes by the Cbl-dependent enzymes. The loss of exogenously added ligand binding or the binding of 125I-protein A occurred with the intestinal basolateral, but not the apical membranes. Based on these results, we suggest that circulatory antibodies to TC II-R cause its in vivo functional inactivation, suppress Cbl uptake by multiple tissues, and thus cause severe Cbl deficiency and the noted failure to thrive.

Gastric intrinsic factor and its receptor

Cbl metabolism has been the subject of many studies since the existence of Cbl was suspected in the first decades of the twentieth century. These studies have confirmed the high complexity of the assimilation of Cbl by the organism. During absorption, Cbl is bound to two glycoproteins, Hc and IF, in a consecutive manner. Over the last few years, it has been demonstrated that Cbl bound to Hc in the stomach is only transferred to IF after the action of pancreatic trypsin. It is also possible that Hc-bound biliary Cbl is transferred to IF in this way and that the Cbl in the Cbl-IF complex is absorbed in the terminal ileum, thus constituting an enterohepatic cycle. Knowledge concerning the sites of synthesis and secretion of IF is becoming more detailed due to the use of immunocytochemistry and in situ hybridization techniques. It is now certain that in man, IF is not only localized in gastric parietal cells, but also in other foregut-derived cells. This observation may explain the multiple physiological stimuli involved in mediating IF secretion. Determination of the molecular structure of purified Cbl binders can be added to the significant progress made due to the application of molecular biology techniques to the field of isolation and structural characterization of cDNA encoding Cbl binders, and particularly IF. Studies of IF, Hc and TC in different species and those into the properties of acceptor fragments have allowed the distinction between the Cbl binding site on IF and the IF-Cbl binding site on the IFCR. The absence of experimental models cause difficulties in studying transcytosis of Cbl through the enterocyte. There are also problems in determining the structure of IFCR as it is difficult to obtain a large quantity of a molecule which denatures very quickly. Studies into IFCR expression in polarized cancerous cells of intestinal or renal origin, including the effects of different pharmacological agents, along with the results of immunochemical investigations are beginning to clarify the pathway involved in the transport of Cbl through the enterocyte.

Membrane expression and interactions of human transcobalamin II receptor

Antiserum raised to purified 62-kDa human placental transcobalamin II receptor (TC II-R) has been used to study its synthesis and membrane expression. The antiserum immunoprecipitated a 45-kDa protein from the cell-free translation using human kidney mRNA and recognized a single 124-kDa band on immunoblotting of placental and other human tissue membranes, and quantitation of the blots revealed high levels of TC II-R expression in the human kidney followed by placenta, intestine, and liver. Triton X-100 extraction of placental membranes resulted in the complete (100%) solubilization of the receptor, and immunoblotting of the Triton X-100-soluble fraction revealed a single band of 62 kDa. Lipid extraction of placental membranes with a mixture of chloroformmethanol (2:1) followed by immunoblotting revealed a single band of molecular mass 62 kDa. The molecular mass of the pure Triton X-100-bound receptor increased on SDS-polyacrylamide gel electrophoresis from 62 to 124 kDa upon its insertion in liposomes prepared using egg phosphatidylcholine and cholesterol. Chemical cross-linking of native membrane-or lipid vesicle-bound TC II-R or detergent-soluble extracts of the membrane with 125I-TC II-cobalamin revealed that both the 124- and 62-kDa forms of the receptor were active in ligand binding. Based on these results we suggest that TC II-R is synthesized as a single polypeptide of 45 kDa, and following its maturation (involving N- and O-glycosylation) the 62-kDa mature receptor is expressed in plasma membranes as a noncovalent dimer of 124 kDa. The dimerization of TC II-R in the plasma membranes is due to its interactions with annular lipids.

Intrinsic factor covalently bound to Sepharose as affinity medium for the purification of a soluble intrinsic factor receptor from human urine

We have identified a soluble receptor for intrinsic factor (IF) in human urine. The purification of this protein by affinity chromatography required a preliminary purification of IF from hog pyloric mucosal extract. This was achieved by thermolabile cobalamin-ethanol-aminohexane Sepharose affinity chromatography with a 133-fold purification, a yield of 45% and a specific binding activity of 15720 pmol/mg protein. The purified Cbl-IF complex was coupled to epoxy-Sepharose with a yield of 23.8% and a specific activity of 1.2 nmol per mol of gel. The soluble IF receptor was purified form 200 ml of urine concentrate of pregnant women. Desorption was performed at pH 5.0 and in the presence of 5 mM EDTA. The soluble IF receptor was purified 17,200-fold with a yield of 52% and a IF binding capacity of 3260 pmol per mg of protein. A single protein with a Mr of 70,000 was found in silver-stained SDS-PAGE.

Pernicious anemia revisited

OBJECTIVE

We discuss the pathophysiologic mechanisms, clinical features, diagnosis, and treatment of pernicious anemia (PA).

DESIGN

A review of the clinical applications of the diagnostic and therapeutic progress of PA is presented.

MATERIAL AND METHODS

A patient with PA may have a wide range of initial complaints that affect various organ systems or may be entirely asymptomatic. Hematologic variables may be normal in patients with cobalamin (Cbl) deficiency. Because of the difficulties in diagnosing Cbl deficiency, alternatives to measuring Cbl have been sought. Determining the urinary methylmalonic acid level is a less invasive, more practical, and, possibly, more sensitive method. The Schilling test is performed for assessment of the absorption of orally ingested radiolabeled crystalline cyano-Cbl; results should be interpreted cautiously.

RESULTS

Vitamin B12 therapy should be lifelong. It is customarily administered intramuscularly. Other routes of administration have been studied.

CONCLUSION

PA is one of the most treatable hematologic disorders.

Intrinsic factor-cobalamin receptor activity in a marsupial, the American opossum (Didelphis virginiana)

1. Significant and specific binding of intrinsic factor-cobalamin occurred in proximal but not in the distal half of the intestine in an adult marsupial, the American opossum. 2. The purified opossum kidney receptor, like rat and canine kidney receptors, revealed a single band of M(r) approximately 230 on SDS-PAGE. However, unlike the rat and canine receptors, the opossum receptor was sensitive to both Endoglycosidase H and peptide-N-glycosidase F. 3. The opossum intrinsic factor-cobalamin receptor demonstrated a ten-fold higher affinity for intrinsic factor-cobalamin complex when the source of IF was from the opossum pancreas, rather than rat stomach. 4. The opossum kidney receptor had low immune-crossreactivity with anti-serum raised to rat and canine kidney receptor. 5. These studies suggest that intrinsic factor-cobalamin receptor expressed in the American opossum, though conserved, appears to be structurally different from the rat and canine receptors.

Regulated expression of intrinsic factor-cobalamin receptor by rat visceral yolk sac and placental membranes

Intrinsic factor-cobalamin receptor (IFCR) activity in visceral yolk sac and placental membranes is regulated during pregnancy in rats. While the IFCR activity declined in the visceral yolk sac membranes by 15-fold, it rose nearly 20-fold in the placental membranes from fourteen to nineteen days of gestation. The visceral yolk sac membranes revealed a 230 kDa protein that co-migrated with pure rat renal IFCR. This 230 kDa band was also identified as IFCR in both the membranes by immunoblotting with anti-serum to rat renal IFCR. Immunoprecipitation of 35S labeled proteins obtained from in vitro translation using visceral yolk sac mRNA from 14-day pregnant rats, yielded on SDS-PAGE a single band of 220 kDa, while those obtained from 19-day pregnant rats did not. The binding of intrinsic factor-cyano[57Co]cobalamin complex to the visceral yolk sac membranes was inhibited by preincubation of these membranes with anti-serum to rat IFCR but not with anti-serum to rat asialoglycoprotein receptor or mannose or mannan or N-acetylglucosamine. Based on these results, we suggest that the IFCR activity, protein expression and mRNA levels in fetal membranes are regulated during pregnancy and may play an important role in the maternal-fetal transfer of cobalamin.

Isolation and sequence analysis of variant forms of human transcobalamin II

Two cDNA clones (1.9 kb and 1.5 kb, respectively) encoding full length human TC II have been isolated from a human endothelial cell cDNA library and sequenced. The differences between the two clones are the length of the 5' end and the 3' end non-coding regions and the codon at position 198 and 219. Both the clones differ from the recently isolated (human endothelial cell) cDNA for TC II (Platica, O., Janecko, R., Quadros, E.V., Regee, A., Romain, R. and Rothenberg, S.P. (1991) J. Biol. Chem. 266, 7860-7863) in codon 259 and 376 and in their calculated pI values. In vitro transcription followed by translation in a reticulocyte lysate system and SDS-PAGE revealed that the isolated cDNA clones encode a protein of 43 kDa. Upon treatment with canine pancreatic microsomes, the molecular mass of the in vitro translated product was reduced to 41.5 kDa, indicating the presence of an approximately 1.5 kDa signal peptide. This translation product was immunoprecipitated with rabbit anti-serum to human TC II and was able to bind to Cbl-Sepharose beads. The amino acid sequence alignment of TC II with that of other Cbl binding proteins (rat intrinsic factor, human transcobalamin I and porcine haptocorrin) revealed only 33% overall homology. However, there were four regions of greater than 80% homology and two regions of about 60% homology. These regions encompass the majority of the hydrophobic areas of the Cbl-binders. Based on these studies, we suggest that structural basis for the expression of different polymorphic forms of TC II may be due to single point mutations and that TC II, like other mammalian Cbl-binders, have evolved from a common ancestral gene. Furthermore, the Cbl-binding functional domain most probably resides in a hydrophobic pocket which is formed by all or some of the six regions of high homology.

Synthesis and secretion of cobalamin binding proteins by opossum kidney cells

Opossum kidney epithelial cells synthesize and secrete two Cobalamin (Cbl) binding proteins of Mr 66,000 and 43,000. When grown on culture inserts, the apical medium contained both these proteins while the basolateral medium contained only the 43 kDa Cbl binder. Colchicine, a microtubule disruptive drug, increased two fold the apical but not the basolateral secretion of the Cbl binding proteins. Although the opossum Cbl binders did not cross react with anti-serum raised to Cbl binders from other species, the identity based on Cbl binding and size suggest that the 66 kDa and 43 kDa proteins are haptocorrin and transcobalamin II.

In vitro translation and expression of renal intrinsic factor-cobalamin receptor

The primary translation product of intrinsic factor (IF)-cobalamin receptor (IFCR) mRNA from rat kidney is a single polypeptide chain of Mr = 215,000. When expressed in Xenopus laevis oocytes the IFCR binding activity is expressed with mRNA of a size between 5 to 7 kb. These results suggest that IFCR mRNA transcripts are present in the renal tissue and encode a single chain, large molecular weight precursor. Furthermore, Xenopus oocytes can be used as a screening system in the expression cloning of the renal IFCR.

Binding assay and physicochemical characteristics of solubilized intrinsic factor receptor in ileal mucosal homogenates using phenyl-Sepharose to separate the saturated receptor from free intrinsic factor

A radioisotopic assay was set to determine the physicochemical properties of the solubilized intrinsic factor receptor in pig mucosal extracts. In this assay, phenyl-Sepharose was used to separate the receptor-intrinsic factor-labelled cobalamin complex from the free saturated intrinsic factor. The association constant (at pH 7.4) of the receptor-intrinsic factor complex was estimated at 3.4 +/- 0.3 nM-1. Adsorption of the apo-receptor to phenyl-Sepharose allowed its binding site to be made accessible to intrinsic factor with an association constant in order of 6 nM-1. The receptor binding activity obtained with five mucosal extracts was closely correlated with that obtained by gel filtration of the intrinsic factor-receptor complex (r = 0.99). The radioisotope assay was used to detect the unsaturated receptor (apo-receptor) in sucrose density ultracentrifugation and in superose 6 gel filtration. The sedimentation coefficient was 9.5 s. The apo-receptor was eluted in three peaks in gel filtration, corresponding to the formation of oligomers. The peak of the monomer was increased in presence of EDTA. Its molecular mass was estimated at 270 kDa and its Stokes radius at 5.9 nm. It was concluded that calcium is involved in the oligomerisation of the apo-receptor.

Renal brush border membrane bound intrinsic factor

A highly active receptor for intrinsic factor (IF)-cobalamin (Cbl) complex has been detected and reported in mammalian kidney earlier (Seetharam, B., et al. (1988) J. Biol. Chem. 263, 4443-4449). The physiological role of this receptor in normal Cbl homeostasis is not known. In addition to binding of exogenously added IF-[57Co]Cbl, the renal apical membranes contain endogenous IF or IF-Cbl. Washing with pH 5/EDTA buffer enhanced the binding of exogenously added IF-[57Co]Cbl to renal apical but not basolateral membranes. The pH 5/EDTA extract from renal apical membranes bound [57Co]Cbl. The complex also bound to rat ileal brush border membrane and promoted ileal transport of [57Co]Cbl. On immunoblots using monospecific antiserum to IF a 62 kDa protein was identified in renal and intestinal apical membranes, serum and in tissue extracts of unperfused rat liver, kidney and heart. The 62 kDa band was eliminated from the renal apical membranes following pH 5/EDTA wash. Rat urine demonstrated unsaturated [57Co]Cbl binding (0.2 to 0.4 pmol/day) of which only 30-40% was immunoprecipitated with anti IF and could be identified on immunoblots. The identification of IF in rat renal apical membranes (160-200 ng/mg protein) and secretion of only traces of IF in urine suggest that the renal IF-Cbl receptor may play a role in sequestering IF/IF-Cbl and prevent urinary loss of Cbl.

Cellular localization of intrinsic factor in pancreas and stomach of the dog

A cobalamin (vitamin B12)-binding protein has recently been identified in canine pancreatic juice which is biochemically, immunochemically and functionally similar to canine gastric intrinsic factor. However, the cellular sources of both this pancreatic intrinsic factor and gastric intrinsic factor in the dog are not known. Antisera raised against canine gastric intrinsic factor have been used to examine the distribution of intrinsic factors in the canine pancreas and stomach. Immunoreactivity was demonstrated in duct cells but not acinar or endocrine cells in the pancreas, and in fundic peptic and pyloric gastric pit cells in stomach. All immunostaining was abolished by preabsorption of the antisera with purified canine gastric and pancreatic intrinsic factors. A cellular source of pancreatic intrinsic factor has not been previously described, and the demonstration of intrinsic factor-like immunoreactivity in two cell types in the canine stomach contrasts with its localization in a single cell type in the gastric mucosa of other mammalian species. Furthermore, immunoreactivity in pancreatic duct cells was detected at much higher dilutions of antisera than those required for staining of peptic and gastric pit cells. This suggests a higher concentration of antigen, and supports previous evidence that the pancrease is a major source of intrinsic factor in the dog.

Purification of intrinsic factor receptor from pig ileum using as affinity medium human intrinsic factor covalently bound to Sepharose

Intrinsic factor receptor was purified from hog ileum using human intrinsic factor covalently bound to Sepharose. A yield of 49.6% and a specific activity of about 2500 pmol/mg protein were achieved. The purified receptor was very unstable: 24 h of storage or addition of sodium phosphate precipitated it. The association constant of the receptor for the cyano[57Co]cobalamin-intrinsic factor complex was estimated to be 2.1 nM-1. In native polyacrylamide gel electrophoresis it resolved in two 256 and 320 kDa bands; beta-mercaptoethanol treatment cleared it into four bands corresponding to molecular masses of 107, 81.8, 63.5 and 53.2 kDa. An additional 39.3 kDa band was considered to be an artefact due to the presence of Triton X-114. Isoelectric focusing polyacrylamide gel electrophoresis resolved the receptor into two isoproteins isoelectric at pH 4.7 and 5.1. A similar result was obtained in column electrofocusing with the 125I-iodinated receptor. The 125I-labelled receptor did not crossreact with rabbit anti-human intrinsic factor antiserum. The electrophoretic properties of the receptor purified with intrinsic factor covalently bound to Sepharose were compared to those of the receptor purified by the use of the classical cobalamin-affinity medium. It was concluded that a disassembled receptor was produced using the classical method.

Lectin binding to the porcine and human ileal receptor of intrinsic factor-cobalamin

The purified porcine receptor for the intrinsic factor-cobalamin complex bound to concanavalin A, lentil lectin and wheat germ lectin covalently coupled to Sepharose and was eluted with the corresponding soluble sugars. In contrast, human intrinsic factor bound efficiently to concanavalin A, to some extent to lentil lectin, but only slightly to wheat germ agglutinin. The binding of IF-Cbl to the receptor was inhibited when the receptor was pre-incubated with soluble wheat germ agglutinin, with an inhibition constant estimated to be 1.9 mumol/l. After transfer of the purified receptor from SDS-PAGE to Immobilon, ligand blotting of the purified receptor with iodinated lectin showed that concanavalin A and lentil lectin bound to three (75, 56 and 43 kDa) components but that wheat germ agglutinin bound only to the 75 kDa component. These results showed that the alpha subunit of the receptor could bind to wheat germ agglutinin, resulting in an inhibition of its binding with intrinsic factor. Both binding sites of intrinsic factor and of wheat germ agglutinin could be located near to each other.

Mapping of the rat liver endothelial membrane with lectins and glycosylated ferritins

We explored the luminal surface of liver sinus endothelium for the presence of lectin receptors and lectinlike substances capable of interacting with specific sugars. We used ferritin-conjugated lectins and glycosylated ferritins as probes. Incubation of small blocks of rat liver with these probes led to the binding of concanavalin A (on A), Ricinus communis (RCA), wheat germ agglutinin (WGA), phytohemagglutinin (PHA) and mannosyl ferritins to the luminal surface of endothelium. Ulex europaeus agglutinin I (UEA), fucosyl, galactosyl, and chitobiosyl-ferritins did not bind. The binding was patchy and sparse in the case of Con A and mannosyl-ferritins but uniform for others. Binding density did not correlate with hemagglutinability of lectins, suggesting that the difference in the hemagglutinability of these lectins did not account for the difference in their binding densities. Bindings were all completely inhibited in the presence of excess specific sugar inhibitors, indicating the specificity of binding. The distribution of binding was segregated on the endothelial membrane, being heaviest on luminal pits. To define the functional significance of this segregated distribution, sinus endothelium was compared to portal-vein endothelium in which endothelial fenestrations are also seen; and these fenestrations as well as pits may be covered by a thin diaphragm. Of interest was the total absence of binding to the diaphragm. The significance of these findings is discussed.