Overview of homocysteine and folate metabolism. With special references to cardiovascular disease and neural tube defects

This overview addresses homocysteine and folate metabolism. Its functions and complexity are described, leading to explanations why disturbed homocysteine and folate metabolism is implicated in many different diseases, including congenital birth defects like congenital heart disease, cleft lip and palate, late pregnancy complications, different kinds of neurodegenerative and psychiatric diseases, osteoporosis and cancer. In addition, the inborn errors leading to hyperhomocysteinemia and homocystinuria are described. These extreme human hyperhomocysteinemia models provide knowledge about which part of the homocysteine and folate pathways are linked to which disease. For example, the very high risk for arterial and venous occlusive disease in patients with severe hyperhomocysteinemia irrespective of the location of the defect in remethylation or transsulphuration indicates that homocysteine itself or one of its “direct” derivatives is considered toxic for the cardiovascular system. Finally, common diseases associated with elevated homocysteine are discussed with the focus on cardiovascular disease and neural tube defects

Methionine: Identification of methylated proteins during neurulation in the rat embryo

Previously, when embryos were cultured without methionine supplements on bovine sera, neural tube proteins were hypomethylated and neural tubes failed to close. To identify the proteins that became methylated, rat embryos were first cultured on methionine deficient bovine serum for 40 hours, then incubated with puromycin for 1 hour, and, finally, incubated with (methyl-\sp{14}C) -methionine for 5 hours. Based on molecular weights, isoelectric points and western immunoblots, the (methyl-\sp{14}C) labeled proteins were identified as actin, $\alpha\beta$-tubulin and neurofilament-L. Indirect immunofluorescence studies indicated that methionine influenced localization of actin and $\alpha\beta$-tubulin in the apical and basal cytoplasm of the elongated neuroepithelial cells. Specifically, without methionine addition localization of these proteins in the basal cytoplasm did not occur and the neuroepithelial cells lost their columnar morphology. Head fold stage rat embryos were prepared for histological examination after 18 and 24 hours of culture. In general, all observations made after 18 hours only became more apparent after 24 hours. Fluorescence from the binding of actin and $\alpha\beta$-tubulin antibodies was localized in the apical and basal cytoplasm of neural ectodermal cells. In the presence of sodium valproate, antibody fluorescence remained present to some extent in the apical cytoplasm but essentially was absent from the basal cytoplasm of most neural ectodermal cells. Sodium valproate caused the cells and their nuclei to lose their columnar morphology becoming round and also caused the underlying basal lamina to break down. When embryos for culture were taken from dams previously given methionine, antibody fluorescence was generally much more intense but again it was localized in the apical and basal cytoplasm of neural ectodermal cells. Most striking, in methionine pre-treated embryos the localization of antibodies to both actin and $\alpha\beta$-tubulin in the polar cytoplasm was maintained even after exposure to sodium valproate. In addition, methionine pre-treatment allowed the elongated morphology of cells and their nuclei to be maintained as well as the integrity of the basal lamina during exposure to sodium valproate.

Homocysteine interference in neurulation: a chick embryo model

Homocysteine interference in neurulation: a chick embryo model. Afman LA, Blom HJ, Van der Put NM, Van Straaten HW. Department of Pediatrics, University Medical Center Nijmegen, The Netherlands. [email protected] BACKGROUND: Periconceptional folic acid supplementation reduces the occurrence and recurrence risk of neural tube defects (NTD). Mothers of children with NTD have elevated plasma homocysteine levels. Administering homocysteine to chick embryos is reported to cause 27% NTD. Therefore, elevated plasma homocysteine levels per se or a disturbed homocysteine metabolism may be teratogenic to the embryo and may interfere with neural tube closure. Our aim was to obtain a chick embryo model to explore the interference of homocysteine in neural tube closure. METHODS: Homocysteine or saline was administered to chick embryos in ovo at 3 hr, 30 hr, and 60 hr of incubation and harvested at 74 hr. Homocysteine was then applied to chick embryos in vitro at a defined time window of four to six somites and followed for 6 hr. RESULTS: Homocysteine administration to chick embryos in ovo resulted in several malformations but not in an increased number of NTDs. Homocysteine administration to chick embryos in vitro resulted in a transient, dose-dependent widening of the anterior neuropore and closure delay of the rhombencephalic neuropore. After 16 hr of incubation the neural tube was closed. CONCLUSIONS: The in vitro chick embryo model appears a good model to explore the interference of a disturbed homocysteine metabolism in neurulatio