Methionine decreases the embryotoxicity of sodium valproate in the rat: in vivo and in vitro observations

Glutathione during embryonic development

BACKGROUND

Glutathione (GSH) is a ubiquitous, non-protein biothiol in cells. It plays a variety of roles in detoxification, redox regulation and cellular signaling. Many processes that can be regulated through GSH are critical to developing systems and include cellular proliferation, differentiation and apoptosis. Understanding how GSH functions in these aspects can provide insight into how GSH regulates development and how during periods of GSH imbalance how these processes are perturbed to cause malformation, behavioral deficits or embryonic death.

SCOPE OF REVIEW

Here, we review the GSH system as it relates to events critical for normal embryonic development and differentiation.

MAJOR CONCLUSIONS

This review demonstrates the roles of GSH extend beyond its role as an antioxidant but rather GSH acts as a mediator of numerous processes through its ability to undergo reversible oxidation with cysteine residues in various protein targets. Shifts in GSH redox potential cause an increase in S-glutathionylation of proteins to change their activity. As such, redox potential shifts can act to modify protein function on a possible longer term basis. A broad group of targets such as kinases, phosphatases and transcription factors, all critical to developmental signaling, is discussed.

GENERAL SIGNIFICANCE

Glutathione regulation of redox-sensitive events is an overlying theme during embryonic development and cellular differentiation. Various stresses can change GSH redox states, we strive to determine developmental stages of redox sensitivity where insults may have the most impactful damaging effect. In turn, this will allow for better therapeutic interventions and preservation of normal developmental signaling. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Dietary methionine intake and neural tube defects in Mexican-American women

BACKGROUND

Nutrients other than maternal folic acid are also thought to play a role in preventing neural tube defects (NTDs). Evidence suggests that methionine interacts with folic acid and vitamin B(12) in the methylation of contractile proteins involved in closing the neural folds. The role of dietary intake of methionine in NTD risk has not been specifically studied among Mexican Americans, a population with one of the highest prevalences of NTDs in the United States.

METHODS

We conducted a case-control study of 184 Mexican American women with NTD-affected pregnancies (case women) and 225 women with normal offspring (control women) who resided along the Texas-Mexico border. The average daily intakes of methionine were calculated from periconceptional food frequency questionnaire data. Women were categorized according to quartiles of daily methionine intake, based on the control mothers' distribution, and the risk for an NTD-affected pregnancy was calculated using the lowest quartile of intake as the referent.

RESULTS

With adjustment for income, body mass index, hyperinsulinemia, and diarrhea, the odds ratios for increasing quartile of methionine intake were: 0.95 (95% confidence interval [CI], 0.48,1.90), 0.92 (95% CI, 0.46,1.84), and 0.66 (95% CI, 0.30,1.45). Some evidence of interaction between dietary methionine and serum vitamin B(12) was noted particularly at higher levels of both components.

CONCLUSIONS

This study was limited by a small sample size but examined this association in an exclusively Hispanic population. Results were suggestive of a potential protective effect for NTDs with increasing maternal dietary methionine intake.

Vitamins, folic acid and the cause and prevention of neural tube defects

Primary prevention of neural tube defects has been demonstrated in humans by maternal therapy with multivitamins and folic acid or folic acid alone. It has also been shown in several animal models of neural tube defects. One of these, the curly tail mouse, has been used extensively to study which agents will prevent neural tube defects in embryos when administered to the mother in early pregnancy. Prevention is achieved with retinoic acid, inositol and the DNA inhibitors hydroxyurea, mitomycin C, 5-fluorouracil and cytosine arabinoside. In no case were neural tube defects prevented in every embryo. A possible preventive effect was seen with riboflavin, vitamin C and vitamin D2. Despite the use of a variety of dose levels, no prevention was achieved with folic acid, folinic acid, Pregnavite Forte F tablets, pyridoxine or vitamin B12, or triamcinolone and cycloheximide (inhibitors of mRNA and protein synthesis, respectively), zinc, homocysteine, methionine and thymidine. Various studies have investigated whether there is a biochemical lesion in folate metabolism in women who have had children with neural tube defects. While there is no difference in their dietary intake of folate compared with control patients, the correlation between their dietary folate and the level of folate in both serum and red blood cells is distorted. Also they are less efficient at raising their folate levels after a folate load. The effects are minor but may hint at a lesion which, if identified, could act as a marker for 'at risk' women.

Spontaneous neural tube defects in splotch mice supplemented with selected micronutrients

Splotch (Sp/Sp) mice homozygous for a mutation in the Pax3 gene inevitably present with neural tube defects (NTDs), along with other associated congenital anomalies. The affected mutant embryos usually die by gestation days (E) 12-13. In the present study, the effect of modifier genes from a new genetic background (CXL-Sp) and periconceptional supplementation with selected micronutrients (folic acid, 5-formyltetrahydrofolate, 5-methyltetrahydrofolate, methionine, myoinositol, thiamine, thymidine, and alpha-tocopherol) was determined with respect to the incidence of NTDs. In order to explore how different exposure parameters (time, dose, and route of compound administration) modulate the beneficial effects of micronutrient supplementation, female mice received either short- or long-term nutrient supplements via enteral or parenteral routes. Embryos were collected on E12.5 and examined for the presence of anterior or posterior NTDs. Additionally, whole mount in situ hybridization studies were conducted in order to reveal/confirm normal expression patterns of the Pax3 gene during neurulation in the wild-type and Sp/Sp homozygous mutant mouse embryos utilized in this study. A strong Pax3 signal was demonstrated in CXL-Sp embryos during neural tube closure (E9.5 to E10.5). The intensity and spatial pattern of expression were similar to other Splotch mutant mice. Of all the micronutrients tested, only supplementation with folic acid or 5-methyltetrahydrofolate rescued the normal phenotype in Sp/Sp embryos. When the folate supplementation dose was increased to 200 mg/kg in the diet, the incidence of rescued splotch homozygotes reached 30%; however, this was accompanied by six-fold increased resorption rate.

Mouse models of neural tube defects: investigating preventive mechanisms

Neural tube defects (NTD), including anencephaly and spina bifida, are a group of severe congenital abnormalities in which the future brain and/or spinal cord fail to close. In mice, NTD may result from genetic mutations or knockouts, or from exposure to teratogenic agents, several of which are known risk factors in humans. Among the many mouse NTD models that have been identified to date, a number have been tested for possible primary prevention of NTD by exogenous agents, such as folic acid. In genetic NTD models such as Cart1, splotch, Cited2, and crooked tail, and NTD induced by teratogens including valproic acid and fumonisins, the incidence of defects is reduced by maternal folic acid supplementation. These folate-responsive models provide an opportunity to investigate the possible mechanisms underlying prevention of NTD by folic acid in humans. In another group of mouse models, that includes curly tail, axial defects, and the Ephrin-A5 knockout, NTD are not preventable by folic acid, reflecting the situation in humans in which a subset of NTD appear resistant to folic acid therapy. In this group of mutants alternative preventive agents, including inositol and methionine, have been shown to be effective. Overall, the data from mouse models suggests that a broad-based in utero therapy may offer scope for prevention of a greater proportion of NTD than is currently possible.

Effect of maternal administration of vigabatrin during late gestation on fetoplacental amino acid profile in the mouse

The objectives were to determine if vigabatrin in late gestation affects fetal growth and causes alterations in amino acid concentrations in the mouse. A single dose of 450 mg/kg VGB in saline or a proportionate volume of saline was administered intraperitoneally (IP) to Theiler outbred (TO) mice on gestation day 15 and maternal plasma, placentae and fetuses were collected at different time intervals after treatment. VGB attained peak concentration in the maternal plasma and the fetus at 2 h after treatment and in the placenta at 4 h. At 12 h significantly lower concentrations of several amino acid including methionine were found in the placentae and fetuses in the treated group. After 24 h, no difference was seen in the plasma amino acid concentrations but in the placentae and fetuses a significant decrease occurred in some amino acids in the treated group. At 48 and 72 h, a generalized increase in most amino acid levels occurred in the fetus and placenta but not in maternal plasma of the treated group although the fetal and placental weights were significantly reduced. VGB during late gestation causes fetal growth retardation accompanied by an initial disruption of amino acid supply followed by an increase in amino acid concentrations for the next 2 days. This increase did not help restore growth suggesting that early fetal period is particularly vulnerable to VGB-induced intrauterine growth retardation in the mouse.

Effects of supplemental L-methionine on E-64 [trans-epoxysuccinyl-1-leucyl-amido (4-guanido) butane]-induced dysmorphology in rat embryos cultured in vitro

E-64 [trans-epoxysuccinyl-1-leucyl-amido (4-guanido) butane] is teratogenic, inducing a spec-trum of malformations in vivo and producing similar effects in vitro. Numerous studies support the concept that E-64-induced malformations result from embryonic nutritionaldeficiency, without affecting the maternal nutritional status. This has provided a useful model with which to investigate the nutritional requirements of the early embryo, as well as the role of various nutrients in the etiology of congenital defects. In the current investigation, we examined effects of L-methionine on E-64-induced embryotoxicity in vitro. For these experiments, we cultured rat embryos 9.5 days postconception (p.c.) for 48 hours with E-64 and/or L-methionine. We found that the addition of L-methionine to E-64-exposed cultures reduced optic abnormality and increased embryo protein. These results suggest that embryopathy largely results from a deficiency of L-methionine although E-64 limits the supply of all amino acids to the embryo. Furthermore, although endocytosis and degradation of proteins by the visceral yolk sac (VYS) supply most amino acids to the embryo, free amino acids may be compensatory when this source is reduced. These results support those of previous investigations that suggest L-methionine is a limiting nutrient for embryonic development.

Effects of methionine on selenium embryotoxicity in cultured rat embryos

Effects of methionine, an essential amino acid, on the embryotoxicity of selenium (Se) were examined using the rat embryo culture. Rat embryos at day 9.5 of gestation were cultured for 48 h in the presence of sodium selenite at 10 and 20 microM or sodium selenate at 30 and 100 microM with or without the addition of 1 mM DL-methionine. Selenite at 20 microM or selenate at 100 microM alone increased the incidence of embryonic malformation and inhibited the embryonic growth. The addition of methionine increased the incidence of embryonic malformation at 10 microM of selenite but decreased the incidence of embryonic malformation at 100 microM of selenate. On the other hand, the addition of methionine partially restored the inhibited embryonic growth at 20 microM of selenite or at 100 microM of selenate. It was considered from these results that the methionine availability in the embryonic environment and the oxidation state of Se are critical in Se embryotoxicity.

A role for protein kinase C and its substrates in the action of valproic acid in the brain: implications for neural plasticity

Valproic acid (VPA) is a broad-spectrum anticonvulsant with well-documented teratogenic effects, but whose mechanism of action is largely unknown. In the present study we have examined the effects of VPA on the expression of two prominent substrates for protein kinase C (PKC) in the brain, MARCKS and GAP-43, which have been implicated in actin-membrane plasticity and neurite outgrowth during neuronal differentiation, respectively, and are essential to normal brain development. Immortalized hippocampal HN33 cells exposed to VPA exhibited reduced MARCKS protein expression and demonstrated increased GAP-43 protein expression, with concomitant alterations in cellular morphology, including an increase in the number and length of neurites and accompanied by a reduction in cell growth rate. The effects of VPA were observed at clinically relevant concentrations following chronic (>1 day) VPA exposure. We also present evidence for a VPA-induced alteration in PKC activity, as well as temporal changes in individual PKC isozyme expression. Inhibition of PKC with the PKC-selective inhibitor, LY333531, prevented the VPA-induced down-regulation of membrane-associated MARCKS, but had no effect on the cytosolic MARCKS reduction or the GAP-43 up-regulation. Inhibition of PKC by LY333531 enhanced the differentiating effects of VPA; additionally, LY333531 alone induced greater neurite outgrowth in this cell line. Collectively, these data indicate that VPA induces neuronal differentiation, associated with a reduction in MARCKS expression and an increase in GAP-43 expression, consistent with the hypothesis that a reduction in MARCKS at the membrane may be permissive for cytoskeletal plasticity during neurite outgrowth.

Perturbations in choline metabolism cause neural tube defects in mouse embryos in vitro

A role for choline during early stages of mammalian embryogenesis has not been established, although recent studies show that inhibitors of choline uptake and metabolism, 2-dimethylaminoethanol (DMAE), and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3), produce neural tube defects in mouse embryos grown in vitro. To determine potential mechanisms responsible for these abnormalities, choline metabolism in the presence or absence of these inhibitors was evaluated in cultured, neurulating mouse embryos by using chromatographic techniques. Results showed that 90%-95% of 14C-choline was incorporated into phosphocholine and phosphatidylcholine (PtdCho), which was metabolized to sphingomyelin. Choline was oxidized to betaine, and betaine homocysteine methyltransferase was expressed. Acetylcholine was synthesized in yolk sacs, but 70 kDa choline acetyltransferase was undetectable by immunoblot. DMAE reduced embryonic choline uptake and inhibited phosphocholine, PtdCho, phosphatidylethanolamine (PtdEtn), and sphingomyelin synthesis. ET-18-OCH3 also inhibited PtdCho synthesis. In embryos and yolk sacs incubated with 3H-ethanolamine, 95% of recovered label was PtdEtn, but PtdEtn was not converted to PtdCho, which suggested that phosphatidylethanolamine methyltransferase (PeMT) activity was absent. In ET-18-OCH3 treated yolk sacs, PtdEtn was increased, but PtdCho was still not generated through PeMT. Results suggest that endogenous PtdCho synthesis is important during neurulation and that perturbed choline metabolism contributes to neural tube defects produced by DMAE and ET-18-OCH3.

Antisense modulation of 5,10-methylenetetrahydrofolate reductase expression produces neural tube defects in mouse embryos

The role of folate metabolism in producing neural tube defects (NTDs) in humans is unknown. In the current study, antisense oligodeoxyribonucleotide technology was utilized to disrupt normal expression of the gene for 5,10-methylenetetrahydrofolate reductase (MTHFR) in organogenesis-stage mouse embryos. Two different antisense probes were microinjected into the amniotic sac of gestation day (GD) 8 mouse embryos with PBS or scrambled sense oligodeoxyribonucleotides injected into control embryos. Concentration-dependent increases in the frequencies of embryos with NTDs were observed for both antisense sequences. The level of mRNA for MTHFR was decreased in embryos treated with the higher concentration of one antisense sequence, indicating that the sequence is able to decrease gene expression. 5-methyltetrahydrofolate, the product of the MTHFR reaction, was able to decrease the incidence of antisense-induced NTDs, but co-injection with L-methionine did not. These results suggest that reduced expression of MTHFR may play a role in producing NTDs.

Effects of supplemental methionine on antiserum-induced dysmorphology in rat embryos cultured in vitro

BACKGROUND

Heterologous antiserum to the visceral yolk sac (AVYS) is teratogenic, inducing a spectrum of malformations in vivo and producing similar effects in vitro. Numerous studies support the concept that AVYS-induced malformations result from embryonic nutritional deficiency, without affecting the maternal nutritional status. This has provided a useful model with which to investigate the nutritional requirements of the early embryo, as well as the role of various nutrients in the etiology of congenital defects.

METHODS

In the current investigation, we examined the effects of methionine and other nutrients on AVYS-induced embryotoxicity in vitro. For these experiments, we cultured rat embryos (9.5 p.c) for 48 hr with AVYS and/or methionine at several concentration levels.

RESULTS

The addition of L-methionine to AVYS-exposed cultures reduced dysmorphology and open neural tube; this effect was concentration dependent. AVYS-induced dysmorphology was completely prevented at a concentration of L-methionine corresponding to 50-fold the basal serum concentration. Utilization of D-methionine, L-leucine, or folic acid (5-methyltetrahydrofolate, MTHF) instead of L-methionine had no protective effects.

CONCLUSIONS

These results suggest that, although AVYS limits the supply of all amino acids to the embryo, embryopathy largely results from a deficiency of methionine. Furthermore, although endocytosis and degradation of proteins by the VYS supplies most amino acids to the embryo, free amino acids may be compensatory when this source is reduced. These results support those of previous investigations that suggest methionine is required for normal NT closure and that methionine is a limiting nutrient for embryonic development.

Pharmacogenetic screening for susceptibility to fetal malformations in women

OBJECTIVE

To present a review of the literature and research on the pharmacogenetics of congenital defects, with a focus on the need for predictive maternal genotype assays.

DATA SOURCE

MEDLINE searches (January 1985-January 1999), past reference reviews, and unpublished research.

STUDY SELECTION

Review of relevant human, animal, and basic science studies.

DATA EXTRACTION

Data on research on polymorphisms, genotyping, cytochrome P450 enzyme systems, epoxide hydrolase, folate metabolism, metabolism of anticonvulsant medications, molecular genetics of neural tube defects, variations in drug metabolism, and environmental exposures were evaluated.

DATA SYNTHESIS

Data synthesis includes not only a review of the literature but suggests ways such data might be used to facilitate the development of maternal genotype assays, with the goal of preventing birth defects.

CONCLUSIONS

Individuals vary in how they metabolize drugs and handle toxic environmental exposures. In an ideal pregnancy, there is no or limited exposure to medications and environmental agents. However, in women with chronic medical conditions such as heart disease and seizures, this is often not possible. Unfortunately, no techniques have been available to identify those at risk in this population. Gene polymorphisms for a specific enzyme may result in an absence or reduction in the level of enzyme activity or in no change at all, with little effect on the structure/function of the gene product(s); they are not associated with clinical phenotypes in either the mother or the fetus. Other polymorphisms may be only markers. Thus, developing genotyping assays for women that are predictive of phenotype expression in the fetus is the key to screening for polymorphisms. As more mutations are identified and clinical, pharmacologic, biologic, and pharmacokinetic relationships are established, using these polymorphisms to develop a genotyping assay for women may become a clinical reality, possibly leading to preventive prepregnancy or prenatal treatment that may play an increasingly effective role in maternal care.

Effects of methionine supplement on methionine incorporation in rat embryos cultured in vitro

The effect of supplementary L-methionine (Met) on the incorporation of methionine was evaluated in 9.5-day rat conceptuses cultured in vitro. Parallel experiments with L-leucine (Leu) were performed for comparison. Conceptuses were cultured for 24 hr in the presence of 3H-labeled Met or Leu, and the incorporation of radiolabel into the embryo and visceral yolk sac was measured. Supplementary Met proportionately increased the incorporation of Met, but supplementary Leu did not have as great an effect on the incorporation of Leu. A hypothesis is presented to explain these findings. It is proposed that Met, but not Leu, is a rate-limiting nutrient for organogenesis-stage rat embryos cultured in rat serum. The results are also discussed with reference to the established efficacy of supplementary folic acid in decreasing the incidence of neural tube defects in human populations and to claims that Met reverses certain teratogenic phenomena, both in vitro and in vivo.

Drug and environmental factors associated with adverse pregnancy outcomes. Part III: Folic acid: pharmacology, therapeutic recommendations, and economics

OBJECTIVE

To review folic acid's mechanism of action, adverse effects, therapeutic recommendations, compliance, and cost.

DATA SOURCES

A MEDLINE search was conducted through December 1997. Additional sources were obtained from Current Contents and citations from the references obtained. Search terms included folate, folic acid, neural tube defect, homocysteine, and methylenetetrahydrofolate reductase.

STUDY SELECTION

Animal and human studies examining the effects of folate were reviewed.

DATA EXTRACTION

Data collected included mechanism of action, safety issues, dosing recommendations, compliance with recommendations, and economics.

DATA SYNTHESIS

Folic acid decreases neural tube defect risk through an effect on methionine-homocysteine metabolism. In addition, increased folate intake may reduce cardiovascular morbidity and mortality. Since toxicity is minimal, everyone can potentially benefit from increased folate consumption. To help achieve this, the Food and Drug Administration has mandated that cereal grain be fortified with 140 micrograms of folic acid per 100 g of grain, which will add approximately 0.1 mg of folate to the average diet. Studies recommend supplementing with 0.2 mg to promote optimal homocysteine concentrations and for preventing neural tube defects.

CONCLUSIONS

Despite fortification, most women will still receive less folate than the 0.4 mg/d recommended by the Public Health Service. All population groups would benefit from increased folate intake. Current studies indicate 200 micrograms/d may be the minimum effective amount of fortification needed for normalizing homocysteine concentrations and preventing a significant number of neural tube defects; thus, a higher level of food fortification may be warranted.

Nutritional studies of the embryo during early organogenesis with normal embryos and embryos exhibiting yolk sac dysfunction

In 1961 we reported that heterologous kidney antiserum when injected into pregnant rats resulted in wide spectrum of congenital malformations. Further studies identified that it was the IgG component of the antiserum that was teratogenic and that complement was not necessary to produce the teratogenic effect. Labeled antibody studies demonstrated that the kidney antiserum localized in the kidney and in the visceral yolk sac (VYS) and parietal yolk sac placentas. Preparation of yolk sac (YS) antiserum proved to be more potent than the kidney antiserum. Adsorption studies with VYS and parietal yolk sac antiserum revealed that the site of the teratogenic process was located in the VYS. In vitro embryo culture experiments demonstrated that direct injection of the teratogenic antibody into the amniotic or YS cavity did not injure the embryo, thus indicating that the teratogenic antibody had to come in contact with the absorptive surface of the VYS. Collaboration with Dr. John Lloyd demonstrated that teratogenic antibody interfered with the process of pinocytosis and the delivery of amino acids (AA) to the developing embryo. Our studies into the nature of the source of AA for the embryo indicated that in some instances > 95% of the AA present in the developing embryo were derived from maternal proteins and the remainder from free AA in the maternal serum. We also demonstrated that embryonic methionine was derived primarily from the digestion of maternal serum proteins but that more of the methionine was diverted from the synthesis of embryonic proteins, supporting the view that it has important functions other than the synthesis of proteins. All these studies focus on the role of the YS in human development and whether human YS dysfunction may play a role in the pathogenesis of congenital malformations. Further studies on the delivery of AA to the embryo are warranted to determine whether certain AA are in short supply in maternal serum and place the embryo at risk if nutritional alterations in the maternal environment occurs. Furthermore, the YS may be an organ whose role might offer opportunities for pregnancy control.

Is dietary intake of methionine associated with a reduction in risk for neural tube defect-affected pregnancies?

Results from experimental animals and other laboratory data have suggested a role for methionine, an essential amino acid, in normal closure of the neural tube. We hypothesized that women who had higher dietary intakes of methionine would be at lower risk for neural tube defect (NTD)-affected pregnancies. Data were derived from a population-based case-control study of fetuses and liveborn infants with NTDs among a 1989-1991 California birth cohort. Interviews, which included a 100-item food frequency questionnaire, were conducted with mothers of 424 NTD cases and 440 nonmalformed controls. Risk for having an NTD-affected pregnancy was estimated according to quartiles (established from intakes among control mothers) of average daily maternal dietary intake of methionine in the 3 months before conception. We observed an approximately 30-40% reduction in NTD-affected pregnancies among women whose average daily dietary intake of methionine was above the lowest quartile of intake (> 1,341.86 mg/ day). These reductions in NTD risk were observed for both anencephaly and spina bifida; remained after adjustment for maternal race/ethnicity and education; and were observed irrespective of maternal level of folate intake. Although we were unable to establish whether the observed reductions in NTD risk were attributable to maternal periconceptional methionine intake or to another highly correlated nutrient, these data add to the growing body of evidence that maternal diet plays a role in neural tube closure.

Hyperhomocysteinaemia and associated disease

An elevated plasma homocysteine level may result from various environmental and genetic factors. Herediatary causes of severe hyperhomo-cysteinaemia are very rare and usually lead to disease in childhood or adolescence. Common pathology consists of early atherosclerotic vascular changes, arterioocclusive complications and venous thrombosis. Mildly elevated genetically determined plasma homocysteine levels are observed in 5% of the general population. In the last two decades research has shown mild hyperhomocysteinaemia to be linked to an increased risk of premature atherosclerosis, pregnancies complicated by neural tube defects and early pregnancy loss, and venous thrombosis. Homozygosity for thermolabile MTHFR deficiency has been identified as one important genetic factor, which expression is modified by dietary folate intake. Although mild hyperhomocysteinaemia can easily be treated by vitamin supplementation the beneficial effects of such treatment remains to be shown.

Rat embryo culture using rabbit serum as a medium for developmental toxicity studies

The usefulness of rabbit serum as a culture medium for postimplantation rat embryos was examined. Rat embryos at day 9.5 of gestation were cultured in a mixture of rat and rabbit sera at various ratios (v/v) for 48 h. In 100% rat serum, a usual medium, rat embryos grew well. On the contrary, rat embryos died with little growth in 100% rabbit serum. In 75% rabbit and 25% rat sera, rat embryos grew but were morphologically abnormal. In 50% rabbit and 50% rat serum, however, rat embryos grew well showing no morphological abnormalities, as in 100% rat serum. It was concluded from these results that rabbit serum could be used at a proportion up to 50% as a medium for postimplantation rat embryo culture in a mixed form with rat serum. The rat embryo culture using rabbit serum as a medium would be useful in developmental toxicity studies, especially those involving species differences and toxicokinetics.

Effects of methionine on the cytoplasmic distribution of actin and tubulin during neural tube closure in rat embryos

Research has previously shown that, without methionine supplements, neural tube proteins of rat embryos cultured on bovine sera were hypomethylated and neural tubes failed to close. In the present study, to identify the proteins that became methylated during neurulation, rat embryos were first cultured on methionine-deficient bovine serum for 40 hr, then incubated with puromycin for 1 hr, and, finally, incubated with [methyl-14C]methionine and puromycin for 5 hr. On the basis of molecular weights, isoelectric points, and Western immunoblots, the methyl-14C-labeled proteins were identified as actin, alpha beta-tubulin, and neurofilament L. Indirect immunofluorescence studies indicated that without the addition of methionine to the culture, localization of actin and alpha beta-tubulin in the basal cytoplasm did not occur and these neuroepithelial cells lost their columnar morphology.

Effects of folate deficiency on embryonic development

While there is strong evidence that folate deficiency including the use of anti-folate drugs in early pregnancy is teratogenic and may lead to a range of serious abnormalities of the developing fetus including intrauterine death, the mechanism(s) for these effects have not yet been delineated. In neural tube defects, there is increasing evidence that marginal folate status exacerbates the effect of an underlying genetic defect in the mother, the fetus, or both. An abnormal relationship between the ingestion of folate and the folate levels in red blood cells has been found in women who have given birth to infants with neural tube defects. Periconceptional folate supplementation has been shown to give effective protection against the development of neural tube defects. The mechanism of the prevention is as yet unknown. However, folic acid will not prevent all cases of neural tube defect. Moreover, neither determinations of periconceptional vitamin profiles (Mooij et al, 1993) nor determinations of methylmalonic acid and homocysteine levels will detect all women at risk. Anencephaly and spina bifida can be identified prenatally by detection of excessive levels of alpha-fetoprotein in amniotic fluid and maternal serum and by ultrasonographic scanning (Wilson and Rudd, 1993). Both genetic counselling and prenatal diagnosis should be offered to women who are recognized to be at high risk. Research into the underlying biology of neural tube defects is of major importance. The identification of underlying genetic defects would allow for genetic testing and better counselling of families at risk for the occurrence of a neural tube defect.

Rat embryo development on human sera is related to numbers of previous spontaneous abortions and nutritional factors

OBJECTIVES

The objectives were to determine (1) if sera from women with histories of spontaneous abortions were teratogenic to cultured embryos more often than were sera of nonaborters, (2) if the teratogenicity could be corrected by adding nutrients to the sera, and (3) if these findings were relevant to reproductive outcomes.

STUDY DESIGN

Rat embryos were cultured for 48 hours on sera from 102 subjects who had experienced spontaneous abortions. Samples from 48 were retested with nutrients added and 10 took dietary supplements, were again tested with embryo cultures, and reported on their pregnancy outcomes.

RESULTS

The frequencies of teratogenic sera increased with numbers of spontaneous abortions (0 to > or = 5) in a manner that did not deviate from linearity (27% to 89%) (chi 2 p > 0.957). Nutrient supplements were added to 48 samples, and 40 were corrected and 10 subjects were given dietary supplement. Sera from six showed improved embryo cultures, and these women completed their pregnancies.

CONCLUSIONS

Rat embryo cultures may provide unique insights into the causes and treatment of spontaneous abortions.