Mice lacking the folic acid-binding protein Folbp1 are defective in early embryonic development

Periconceptional folic acid supplementation reduces the occurrence of several human congenital malformations, including craniofacial, heart and neural tube defects. Although the underlying mechanism is unknown, there may be a maternal-to-fetal folate-transport defect or an inherent fetal biochemical disorder that is neutralized by supplementation. Previous experiments have identified a folate-binding protein (Folbp1) that functions as a membrane receptor to mediate the high-affinity internalization and delivery of folate to the cytoplasm of the cell. In vitro, this receptor facilitates the accumulation of cellular folate a thousand-fold relative to the media, suggesting that it may be essential in cytoplasmic folate delivery in vivo. The importance of an adequate intracellular folate pool for normal embryogenesis has long been recognized in humans and experimental animals. To determine whether Folbp1 is involved in maternal-to-fetal folate transport, we inactivated Folbp1 in mice. We also produced mice lacking Folbp2, another member of the folate receptor family that is GPI anchored but binds folate poorly. Folbp2-/- embryos developed normally, but Folbp1-/- embryos had severe morphogenetic abnormalities and died in utero by embryonic day (E) 10. Supplementing pregnant Folbp1+/- dams with folinic acid reversed this phenotype in nullizygous pups. Our results suggest that Folbp1 has a critical role in folate homeostasis during development, and that functional defects in the human homologue (FOLR1) of Folbp1 may contribute to similar defects in humans.

Rectal sleeve advancement: repair of rectovaginal fistula associated with anorectal stricture in Crohn's disease

Successful repair of rectovaginal fistula in patients with Crohn's disease has been reported when these patients have a normal appearing rectum. We report the performance of stricturectomy in conjunction with circumferential rectal sleeve advancement for patients who have a rectovaginal fistula arising from an anorectal stricture secondary to Crohn's disease. This technique provides for repair of rectovaginal fistula and correction of the anal stricture and maintains continence.

Lack of association between mutations in the folate receptor-alpha gene and spina bifida

Defects of neural tube closure are among the most common of all human malformations. Epidemiological and genetic studies indicate that most of these defects are multifactorial in origin with genetic and environmental causes. Although periconceptional supplementation of the maternal diet with folic acid has been shown to reduce the recurrence and occurrence of neural tube defects (NTDs) by up to 70%, the underlying mechanism remains unknown. Folic acid enters cells of certain tissues via a receptor-mediated process known as potocytosis. The folate receptor alpha (FR-alpha) gene codes for the protein responsible for binding folate, which is the first, and only, folate-dependent step in folate transport. The FR-alpha exons, which code for mature protein and the intron-exon boundaries, were examined for mutations in three separate studies. Initial screening was performed by single-stranded conformational polymorphism (SSCP) analysis in a subset of 1,688 samples obtained from a population-based case-control study of NTDs in California. In the second study, the DNA sequence of exons 5 and 6 was determined in a group of 50 NTD affected individuals. The final experiment involved using dideoxy fingerprinting (ddF) to screen a population-based case-control sample of 219 individuals who were stratified into four sub-groups on the basis of folate intake and pregnancy outcome. No polymorphism was detected in any of the four exons examined. It is unlikely that the beneficial effects of maternal folate supplementation in preventing NTDs acts through a mechanism involving pharmacological correction of a variant form of folate receptor alpha.

Cholera: calamitous past, ominous future

From the pandemics of the 19th century to the recent disaster in Goma, Zaire, cholera has left an indelible mark on human and medical history. Cholera pandemics in the 19th and 20th centuries drove the development of epidemiology as a serious science. Cholera has continued to press advances in the concepts of disease ecology, basic membrane biology, and transmembrane signaling and in the application of scientific information to treatment design. Furthermore, the lessons learned from the study of pandemic cholera are likely to provide insights into the best means of stopping other pandemics. In spite of tremendous scientific and clinical progress, however, the seventh pandemic has lasted 33 years, and the eighth pandemic appears to have started.

Folate receptors targeted to clathrin-coated pits cannot regulate vitamin uptake

Potocytosis is an endocytic process that is specialized for the internalization of small molecules. Recent studies on the uptake of 5-methyltetrahydrofolate by the folate receptor have suggested that the glycosyl-phosphatidylinositol anchor on this protein causes it to cluster and be internalized by caveolae instead of coated pits. To test this hypothesis directly, we have constructed a chimeric folate receptor that has the glycosyl-phosphatidylinositol anchor replaced with the transmembrane domain and cytoplasmic tail of the low density lipoprotein receptor. The cells with wild-type receptors delivered 5-methyltetrahydrofolate to the cytoplasm more rapidly than did cells expressing the chimeric receptor. This suggests that efficient delivery to the cytoplasm depends on caveolae. In sharp contrast to cells with wild-type folate receptors, cells internalizing folate by clathrin-coated pits were unable to decrease vitamin uptake when they were either folate replete or confluent.

Expression of lactase-phlorizin hydrolase in sheep is regulated at the RNA level

Lactase-phlorizin hydrolase (LPH) is expressed on the intestinal brush border and is responsible for the hydrolysis of lactose, the chief sugar in mammalian milk. The enzyme activity of LPH peaks soon after birth in most mammals and declines to much lower levels before adolescence. The molecular basis of this pattern of expression has not been clearly established. We have measured relative amounts of LPH mRNA in intestine from sheep with ages across a developmental spectrum, including third trimester fetal lambs, newborn lambs and adult sheep. LPH mRNA levels in the jejunum decline approximately 50-fold between infancy and adulthood, in parallel with the reduction in both lactase specific activity and immunologically reactive lactase protein expression in sheep jejunum. LPH mRNA is present in high concentration in the duodenum of newborn lambs, but steadily declines by day 34 and is dramatically reduced in adults. Because the changes in LPH mRNA, protein, and enzymic activity are generally parallel, we conclude that the developmental regulation of LPH in sheep is probably mediated primarily at the mRNA level.

The polymorphic expression of lactase in adults is regulated at the messenger RNA level

BACKGROUND/AIMS

Lactase phlorizin hydrolase (LPH) activity is high in infants but declines 80%-90% before adulthood in most mammals, including humans. However, 95% of whites show autosomal dominant inheritance of a lifelong high lactose digesting capacity (LDC). This study attempted to clarify the molecular mechanism(s) of this phenomenon (posttranslational vs. pretranslational).

METHODS

A race- and sex-balanced cohort (n = 20) was studied, and lactose tolerance and levels of jejunal lactase protein, activity, and messenger RNA (mRNA) were measured.

RESULTS

These data confirm that black heritage predicts low LDC, and white heritage predicts high LDC. Lactase breath hydrogen and determination of lactase/sucrase ratio (L/S) from jejunal biopsy specimens divide the group by high and low LDC phenotypes concordantly. All subjects with an L/S ratio > 0.5 had immunodetectable LPH protein and measurably higher LPH mRNA levels than the remaining subjects. LPH mRNA levels are highly correlated with lactase specific activity (r = 0.80) and L/S ratio (r = 0.88).

CONCLUSIONS

The direct correlation between LPH mRNA levels and lactase expression argues that the gene responsible for the human lactase polymorphism regulates the level of LPH mRNA.

The arts of war and medicine: a study in symbiosis

Practitioners of warfare and medicine have both considered their respective disciplines part art and part science. Each has seen dramatic advances over the past several centuries, and the histories of progress in warfare and medical practice are inextricably intertwined. For example, the congregation of large armies inspired the development of epidemiologic analysis and preventive medicine, which forestalled the disastrous loss of life that typified all wars through the U.S. Civil War. Warfare also helped to clarify the critical distinction between medicine and health in several ways. Battles generate tremendous demands on the trauma surgeon, which triggers advances in understanding of trauma, surgery, fluid and electrolyte management, first aid, and triage. Similarly, the health of soldiers before and after combat is a result of public health measures that physicians have been ill-suited to manage. The dramatic severity of epidemics in armies progressively forced health professionals and politicians to seek solutions to illnesses that had plagued the general population to lesser degrees for millennia. The exigencies of war inspired creation of the nursing profession, with the burden and opportunity falling on women. Women were not allowed to hold positions of responsibility in caring for the sick until the enormity of the Civil War prevented men from occupying all such positions. Because each generation tends to view itself as "modern," its inherent weaknesses often go uncorrected and even unobserved. The interconnected histories of war and medicine provide a warning to remain open to discovering those practices that need radical reform to prevent the current generation of physicians from appearing utterly ridiculous to physicians 100 years hence. This treatise focuses, therefore, on the symbiotic advances in warfare on health before this century because death in war due to trauma was statistically far less important than death due to disease.

Folate receptor allows cells to grow in low concentrations of 5-methyltetrahydrofolate

The folate receptor mediates the uptake of 5-methyltetrahydrofolate in certain cultured cells by a process called potocytosis. When these cells are grown in physiological concentrations of folate, the receptor increases the efficiency of vitamin uptake by 30-fold. We now show that PAM 212 cells, a mouse keratinocyte cell line, are unable to grow in 1 nM 5-methyltetrahydrofolate unless they express a functional folate receptor. These results suggest that under certain conditions, tissue cells in the body may depend on the folate receptor to obtain enough 5-methyltetrahydrofolate for growth.

Expression of a full-length cDNA coding for human intestinal lactase-phlorizin hydrolase reveals an uncleaved, enzymatically active, and transport-competent protein

Lactase-phlorizin hydrolase (LPH) (EC 3.2.1.23/62) is a major intestinal microvillar membrane glycoprotein that digests lactose, the main carbohydrate of milk. To investigate structure/function relationships of LPH and to assess the impact of intracellular processing on the function of LPH and on its transport to the cell surface, we have expressed a full-length cDNA encoding LPH in mammalian COS-1 cells. Analysis of the expressed protein by immunoprecipitation with monoclonal anti-LPH antibodies and treatments with endo-beta-N-acetylglucosaminidase H and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two polypeptides with apparent molecular masses of 215 and 230 kDa, representing the mannose-rich (pro-LPHh) and complex (pro-LPHc) glycosylated forms of the precursor. By contrast to pro-LPH in human enterocytes, the expressed pro-LPH in COS-1 cells does not undergo intracellular proteolytic cleavage to generate a form similar to the mature enzyme of the brush-border membrane. Intracellular cleavage, however, is not essential for the molecule to acquire its enzymatic activity since pro-LPH in COS-1 cells is enzymatically as active as LPH isolated from intestinal brush-border membranes. Indirect immunofluorescent staining of transfected cells demonstrated that pro-LPH is expressed at the cell surface. This was further corroborated by the sensitivity of the complex glycosylated form (pro-LPHc) to trypsin in the medium. Our results provide the first conclusive evidence that pro-LPH is an enzymatically active molecule and that the intracellular proteolysis of pro-LPH is not essential for the generation of transport-competent forms of LPH.

Complete amino acid sequence of human cartilage link protein (CRTL1) deduced from cDNA clones and chromosomal assignment of the gene

Little is known about the primary amino acid structure of human cartilage link protein (CRTL1). We screened a human genomic library with a cDNA encoding the 3' untranslated region and the adjoining B1 domain of chicken link protein. One clone was isolated and characterized. A 3.5-kb EcoRI-KpnI fragment from this genomic clone that contains the human B1 exon was used to map the gene to chromosome 5q13----q14.1. The same fragment was used to screen a cDNA library prepared from mRNA of Caco-2, a human colon tumor cell line. Two overlapping clones were isolated and shown to encode all of CRTL1. The deduced amino acid sequence is 354 residues long. The amino acid sequence shows a striking degree of identity to the porcine (96%), rat (96%), and chicken (85%) link protein sequences. Furthermore, there is greater than 86% homology between the 3' untranslated region of the genes encoding human and porcine link proteins. These results indicate that there has been strong evolutionary pressure against changes in the coding and 3' untranslated regions of the gene encoding cartilage link protein.

Complementary DNA for the folate binding protein correctly predicts anchoring to the membrane by glycosyl-phosphatidylinositol

Membrane bound and soluble forms of a high-affinity folate binding protein have been found in kidney, placenta, serum, milk, and in several cell lines. The two forms have similar binding characteristics for folates, are immunologically cross-reactive and based upon limited amino acid sequence data, are nearly identical. Based upon pulse-chase experiments, a precursor-product relationship has been suggested. The membrane form has been shown to mediate the transport of folate in cells grown in physiological concentrations of folate. A function for the soluble form has not yet been identified. We constructed a cDNA library from a human carcinoma cell line, Caco-2, which expresses the membrane form abundantly. The library was screened and a near full-length cDNA for the folate binder was isolated. Transfection of COS cells with the cDNA inserted in an expression vector resulted in marked overexpression of a membrane-associated folate binder as assessed by direct binding of radiolabeled folate and by indirect immunofluorescence. The deduced amino acid sequence is not consistent with a typical membrane spanning domain but rather with a signal for anchoring via a glycosyl-phosphatidylinositol linkage. Release of the binder with a phosphatidylinositol-specific phospholipase C strongly supports this hypothesis.

Oncogenes in retroviruses, malignancy, and normal tissues

Oncogenes are genes with a proven cancer association that appear to function primarily in the regulation of cellular proliferation and differentiation. The preliminary work leading to their discovery, their association with retroviruses, the retrovirus life cycle, and the use of retroviruses to find and characterize oncogenes is discussed in detail. Chromosomal abnormalities are discussed using Burkitt's lymphoma and the myc oncogene as prototypes. Finally, the normal (nonmalignant) functioning of oncogenes is discussed in terms of embryogenesis and hepatic regeneration.