Teratogen-induced apoptosis may be affected by immunopotentiation

Reduction in valproic acid-induced neural tube defects by maternal immune stimulation: role of apoptosis

Teratogenic deregulation of apoptosis during development is a possible mechanism for birth defects. Administration of valproic acid (VA) during first trimester of pregnancy causes neural tube defects (NTDs). Nonspecific stimulation of the mother's immune system has been shown to reduce various teratogen-induced fetal malformations including NTDs in rodents. This present study investigated the role of reduced apoptosis by maternal immune stimulation in prevention of VA-induced NTDs in CD-1 mice. Prevention of VA-induced NTDs by nonspecific maternal immune stimulation using IFNγ was employed to evaluate the role of reduced apoptosis by IFNγ in this protective mechanism. Apoptosis was quantified using flow cytometry. Terminal Transferase dUTP Nick End Labeling assay was used to localize the apoptosis. Increased apoptosis, suggesting involvement in VA teratogenicity, was observed along the neural tube in both normal and abnormal embryos from VA-exposed dams. Increased apoptosis in normal VA-exposed embryos suggests that VA may alter other cellular processes such as cell proliferation and differentiation in addition to apoptosis. Apoptotic levels in embryos with closed neural tubes from IFNγ + VA dams were similar to controls indicating resistance to VA-induced apoptosis and protection against teratogenicity of VA. In IFNγ + VA exposed embryos with open neural tubes, maternal immune stimulation failed to regulate apoptosis resulting in an NTD. Overall, these results suggest that VA alters several biological processes including apoptosis in the developing embryos to induce fetal malformations. Resistance to VA-induced apoptosis in embryos resulting from maternal immune stimulation may be involved in protective mechanism.

Methylnitrosourea induces neural progenitor cell apoptosis and microcephaly in mouse embryos

BACKGROUND

Prenatal exposure to methylnitrosourea (MNU), an alkylating agent, induces microcephaly in mice. However, its pathogenetic mechanism has not been clarified, especially that in the development of the cerebral cortex.

METHODS

ICR mice were treated with MNU at 10 mg/kg intraperitoneally on day 13.5 or 15.5 of gestation, and the embryos were observed histologically 24 hr after treatment with MNU or at term. To clarify the pathogenesis of microcephaly and histological changes, especially apoptosis, neurogenesis, and neural migration/positioning, we performed histological analysis employing a cell-specific labeling experiment using thymidine-like substances (BrdU, CldU, and IdU) and markers of neurons/neural stem cells.

RESULTS

Histological abnormalities of the dorsal telencephalon, and the excessive cell death of proliferative neural progenitor/stem cells were noted in the MNU-treated embryos. The highest frequencies of cell death occurred at 36 hr after MNU treatment, and little or no neurogenesis was observed in the ventricular zone of the dorsal telencephalon. Abnormality of the radial migration was caused by the reduction of radial fibers in the radial glias. Birth-date analysis revealed the abnormal positioning of neurons and aberrant lamination of the cerebral cortex.

CONCLUSIONS

Our data suggest that prenatal exposure to MNU induces the excessive cell death of neural precursor/stem cells, and the defective development of the cerebral cortex, resulting in microcephalic abnormalities.

Maternal Immunopotentiation Affects Caspase Activation and NF-kappaB DNA-binding Activity in Embryos Responding to an Embryopathic Stress

PROBLEM

Increased embryonic resistance to teratogenic stresses as a result of maternal immunopotentiation is associated with a decrease in the intensity of teratogen-induced apoptosis in target embryonic structures. These findings suggest that this effect of maternal immunopotentiation might be realized through modification of the expression of molecules regulating the teratogen-induced apoptotic process. To examine this possibility, we evaluated caspases 3, 8 and 9 activation as well as nuclear factor (NF)-kappaB DNA-binding activity in the embryos of immunopotentiated mice exposed to cyclophosphamide (CP).

METHODS OF STUDY

The rate of resorptions and the proportion of malformed fetuses in CP-treated mice were recorded on day 19 of pregnancy. Activity of caspases was tested in cytoplasmic extracts collected from the embryonic brain 24 hr after CP treatment using appropriate fluorometric kits, whereas NF-kappaB DNA-binding activity was evaluated in nuclear extracts using the electrophoretic mobility shift assay.

RESULTS

As in our previous studies, immunopotentiated CP-treated females exhibited a lower rate of resorptions or fetuses with open eyes than their non-immunopotentiated counterparts. In parallel, we observed that maternal immunopotentiation normalized the CP-induced activation of the tested caspases as well as the CP-induced suppression of NF-kappaB DNA-binding activity.

CONCLUSIONS

As caspases act as inducers of apoptosis, and NF-kappaB acts in CP-treated embryos as an apoptosis suppressor, the above results suggest that maternal immunopotentiation might affect embryonic sensitivity to embryopathic stresses via NF-kappaB- and caspases-associated pathways.

Reduced birth defects caused by maternal immune stimulation in methylnitrosourea-exposed mice: association with placental improvement

BACKGROUND

Methylnitrosourea (MNU) is a potent carcinogen and teratogen that is associated with central nervous system, craniofacial, skeletal, ocular, and appendicular birth defects following transplacental exposure at critical time points during development, and preliminary studies have suggested that nonspecific maternal immunostimulation may offer protection against development of these birth defects.

METHODS

Our study examined morphologic alterations in fetal limb and digital development and placental integrity following maternal exposure to MNU on GD 9 in CD-1 mice, and characterized the improvement in placental integrity and abrogation of fetal defects following maternal immune stimulation with interferon-gamma (IFN-gamma) on GD 7.

RESULTS

Fetal limbs were significantly shortened (p < 0.0001) and incidence of limb and digital defects (syndactyly, polydactyly, oligodactyly, clubbing, and webbing) was dramatically increased following mid-gestational maternal MNU exposure. Maternal immune stimulation with IFN-gamma on GD 7 lessened incidence of fetal limb shortening and maldevelopment on GD 12 and 14. Further, disruption of placental spongiotrophoblast integrity, increased cell death in placental trophoblasts with increased intercellular spaces in the spongiotrophoblast layer and minimal inflammation, and increased loss of fetal labyrinthine endothelial cells from MNU-exposed dams suggested that MNU-induced placental breakdown may contribute to fetal limb and digital maldevelopment. MNU + IFN-gamma was associated with diminished cell death within all layers of the placenta, especially in the labyrinthine layer.

CONCLUSIONS

These data verify improved distal limb development in MNU-exposed mice as a result of maternal IFN-gamma administration, and suggest a link between placental integrity and proper fetal development.

TNF-alpha protects embryos exposed to developmental toxicants

BACKGROUND

Tumor necrosis factor alpha (TNF-alpha) has been implicated in mediating post-implantation embryo loss or the embryonic maldevelopment induced by development toxicants or maternal metabolic imbalances. In order to clarify the role of TNF-alpha further, a comparative study was performed in TNF-alpha, knockout and TNF-alpha, positive mice, exposed to a reference teratogen, cyclophosphamide (CP).

METHODS

Cyclophosphamide was injected on day 12 of pregnancy and 18-day fetuses were examined for external structural anomalies. Apoptosis and cell proliferation were measured by TdT-mediated biotin-dUTP nick-end labeling and 5'-bromo-2'-deoxyuridine incorporation, respectively, in the brain (an organ, sensitive to the teratogen) of embryos 24 hr after CP injection. NF-kappaB DNA-binding activity by electrophoretic mobility shift assay (EMSA) and the expression of Re1lA (an NF-kappaB subunit) and I(kappa)B(alpha) proteins by Western blot analysis were assessed in the brain of embryos tested 24 and 48 hr after CP treatment.

RESULTS

Surprisingly, the proportion of fetuses with craniofacial, trunk and severe limb reduction anomalies were significantly higher in TNF-alpha -/- females, than in TNF-alpha,+/+ mice. Excessive apoptosis and suppression of cell proliferation was found in the brain, and they were more prominent in TNF-alpha -/- than TNF-alpha +/+ embryos, when examined 24 hr after CP injection. Finally, CP-induced suppression of NF-kappaB DNA-binding activity was found to be enhanced in the brain of TNF-alpha -/- embryos, and the restoration of NF-kappaB DNA-binding activity was compromised.

CONCLUSION

This work demonstrates for the first time that TNF-alpha may act as a protector of embryos exposed to teratogenic stress. One possible mechanism may be restoration of NF-kappaB activity in embryonic cells surviving the teratogenic insult.

Potentiation of the maternal immune system may modify the apoptotic process in embryos exposed to developmental toxicants

PROBLEM

We have previously shown that teratogen-induced embryonic maldevelopment may result from excessive apoptosis in affected organs, but the mechanisms underlying this process are not well understood. Here we investigate the ability of maternal immunopotentiation to affect the apoptotic process and its regulatory genes p53 and bcl-2 in embryos exposed to a teratogenic insult.

METHOD OF STUDY

Potentiation of the immune system in pregnant females was performed with xenogeneic rat splenocytes or with granulocyte macrophage-colony stimulating factor (GM-CSF). The animals were exposed to cyclophosphamide (CP) and the reproductive performance in the various experimental groups was recorded. The level of apoptosis was assessed in the embryonic head and liver by TdT-mediated dUTP-biotin nick end labeling and fluorescence-activated cell sorter (FACS) analysis, while p53 and bcl-2 expression was evaluated by FACS and immunohistochemistry.

RESULTS

In CP-treated females, a decrease in embryonic weight and an increase in the resorption rate and the percentage of embryos exhibiting head malformations were noted. These effects of CP were accompanied by the appearance of apoptotic cells in the head but not in the liver and an increased expression of p53 in embryonic organs, while bcl-2 expression was found to be decreased in the head and increased in the liver. Immunopotentiation with rat splenocytes or GM-CSF was shown to partially normalize the teratogenic effect of CP. It was also found to partially decrease the CP-induced apoptotic process and exhibited a tendency to normalize the expression of p53 and bcl-2 in the embryonic head and liver.

CONCLUSION

Our results suggest a possible role for maternal immunopotentiation in protecting the embryo from teratogenic insults, possibly through regulation of the CP-induced apoptotic process and the expression of p53 and bcl-2.

Maternal immune stimulation in mice decreases fetal malformations caused by teratogens

For unknown reasons, non-specific stimulation of the maternal immune system in pregnant mice has what appears to be a broad-spectrum efficacy for reducing birth defects. Immune stimulation by diverse procedures has proven effective, including footpad injection with Freund's complete adjuvant (FCA), intraperitoneal (IP) injection with inert particles to activate resident macrophages, IP injection with attenuated Bacillus Calmette-Guerin (BCG), and intrauterine injection with allogeneic or zenogeneic lymphocytes. Morphologic lesions that were significantly reduced included cleft palate and associated craniofacial defects, digit and limb defects, tail malformations, and neural tube defect (NTD). Teratogenic stimuli to induce these lesions included chemical agents (2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD], ethyl carbamate [urethane], methylnitrosourea [MNU], cyclophosphamide [CP], and valproic acid [VA]), physical agents (X-rays, hyperthermia), and streptozocin (STZ)-induced diabetes mellitus. Limited information is available regarding mechanisms by which such immune stimulation reduced fetal dysmorphogenesis. The collective literature suggests the possibility that immunoregulatory cytokines of maternal origin may be the effector molecules in this phenomenon.

Nonspecific stimulation of the maternal immune system. I. Effects On teratogen-induced fetal malformations

BACKGROUND

Maternal immune stimulation has reported, but unconfirmed, efficacy for reducing chemical-induced morphologic defects in mice.

METHODS

Teratogenic chemicals (2,3,7, 8-tetrachlorodibenzo-p-dioxin [TCDD], ethyl carbamate [urethane], methylnitrosourea [MNU], or valproic acid [VA]) were given to pregnant mice to induce cleft palate (TCDD, urethane), digital defects (urethane, MNU), or exencephaly (VA). Before teratogen administration, the immune system of female mice was stimulated by intraperitoneal (IP) administration of pyran copolymer or attenuated bacillus Calmette Guérin (BCG), or by footpad injection with Freund's complete adjuvant (FCA).

RESULTS

Fetal defects caused by all four chemicals studied were reduced by maternal immunostimulation, sometimes dramatically. In addition to reducing VA-induced exencephaly, immunostimulation with FCA resulted in fetal mice displaying anury (absence of tails). Activated maternal immune cells could not be detected in fetal circulation using flow cytometry and a fluorescent cell-tracking probe.

CONCLUSIONS

For the chemicals tested, maternal immune stimulation has efficacy in reducing fetal defects. Immune protection against teratogenesis may be an indirect effect of maternal immune cell activation.

The role of apoptosis in normal and abnormal embryonic development

Programmed cell death or apoptosis is a widespread biological phenomenon. Apoptosis is characterized by typical cell features such as membrane blebbing, chromatin condensation, and DNA fragmentation. It involves a number of membrane receptors (e.g., Fas, TNFR) and a cascade of signal transduction steps resulting in the activation of a number of cysteine proteases known as caspases. Disordered apoptosis may lead to carcinogenesis and participates in the pathogenesis of Alzheimer disease, Parkinson disease, or AIDS. Programmed cell death plays an important role in the processes of gamete maturation as well as in embryo development, contributing to the appropriate formation of various organs and structures. Apoptosis is one of the mechanisms of action of various cytotoxic agents and teratogens. Teratogen-induced excessive death of embryonic cells is undoubtedly one of the most important events preceding the occurrence of structural abnormalities, regardless of their nature. Therefore understanding the mechanisms involved in physiological as well as in disturbed or dysregulated apoptosis may lead to the development of new methods of preventive treatment of various developmental abnormalities. The present review summarizes data on the mechanisms of programmed cell death and concentrates on apoptosis involved in normal or disturbed gametogenesis and in normal and abnormal embryonic development.

Maternal immunopotentiation affects the teratogenic response to hyperthermia

Immune responses occurring between the embryo and mother have been shown to influence the embryo's tolerance to teratogens, including chemical teratogens and diabetes-induced teratogenic insult. In this study, we tried to evaluate whether maternal immunostimulation alters the embryo's response to heat shock, one of few teratogens which directly affect the embryo. In order to induce structural anomalies, both intact ICR female mice and mice which had been immunostimulated with xenogeneic rat splenocytes before mating, were exposed to two consecutive exposures to heat (43.6 +/- 0.2 degrees C) for 10 min on day 9 of pregnancy. The number of malformed fetuses, resorptions, and fetal weight were assessed on day 19 of pregnancy. Heat shock-induced apoptosis, and the level of heat shock protein (HSP) 60 expression, were examined in embryonic cells at different time points within 24 h after heating. All these indices differed dramatically in immunized and non-immunized heat shocked females. Heat shocked non-immunized females demonstrated an increased level of resorptions (approximately, 21% versus 8.6% in controls) and the proportion of fetuses with such anomalies as encephalocele and open eyes reached 28% and 21%, respectively. Maternal immunostimulation was associated with a significant decrease in the proportion of fetuses with encephalocele (12.8%), open eyes (8.9%), and resorptions (8%). The maximum level of heat shock-induced apoptosis in cell populations from the embryos of non-immunized females, was approximately, 30% versus 7% in cells of embryos of immunized mice. Heat shock was also followed by a significant increase in HSP60 expression, but only in the cells of embryos of non-immunized females. Together, these findings suggest that the tolerance of mouse embryos to a heat shock-induced teratogenic insult may, to some extent, depend on the character of the maternal immune responses.

TNF-alpha expression in embryos exposed to a teratogen

PROBLEM

The role of tumor necrosis factor (TNF)-alpha produced by embryonic cells in normal and abnormal development is poorly understood. To assess to what extent TNF-alpha may be involved in the process of induced dysmorphogenesis, the expression of TNF-alpha and TNF-alpha receptor (TNFRI) mRNA as well as TNF-alpha protein was evaluated in embryos responding to a cyclophosphamide (CP)-induced teratogenic insult. The effect of maternal immunostimulation increasing the embryo's tolerance to CP on TNF-alpha expression was also investigated.

METHOD OF STUDY

ICR female mice were treated intraperitoneally with 40 mg/kg CP on day 12 of pregnancy. The immunostimulator, xenogeneic rat splenocytes, was injected intrauterine 21 days before mating. Embryos were collected on days 13, 14, or 15 of pregnancy. TNF-alpha mRNA, TNFRI mRNA, and TNF-alpha protein expression were evaluated by in situ hybridization and immunostaining techniques in control, teratogen-treated, and immuno-stimulated teratogen-treated embryos.

RESULTS

CP-treated embryos showed severe external brain and craniofacial anomalies already visible on day 14 of pregnancy. TNF-alpha mRNA transcripts were detected in cells of the brain and the head of 13-day embryos, which preceded the occurrence of CP-induced external craniofacial anomalies. On day 15 of pregnancy, when severe craniofacial anomalies increased, a significant increase in the intensity of TNF-alpha, TNFR1 mRNA transcripts, and TNF-alpha protein expression were observed in cells of the malformed regions of the head and the brain. In other nonmalformed organs of CP-treated embryos such as the liver (not macroscopically different from controls), neither TNF-alpha nor TNFR1 transcripts were detected. Immunostimulation substantially diminished the severity of CP-induced brain and craniofacial anomalies, decreased the resorption rate, and was associated with decreased intensity of TNF-alpha mRNA transcripts detected on day 15 of pregnancy in the head and the brain of CP-treated embryos.

CONCLUSIONS

TNF-alpha expressed in the embryo may be one of the molecules promoting the formation of CP-induced brain and craniofacial anomalies. The decrease of TNF-alpha expression in embryos of immunostimulated females may be one of the mechanisms responsible for the increased tolerance to the teratogenic insult.

TNF-α Messenger RNA and Protein Expression in the Uteroplacental Unit of Mice with Pregnancy Loss

An elevated expression of TNF-α in embryonic microenvironment was found to be associated with postimplantation loss. In this work, we examined the pattern of TNF-α expression at both the mRNA and the protein level as well as the distribution of TNF-α receptor mRNA in the uteroplacental unit of mice with induced (cyclophosphamide-treated) or spontaneous (CBA/J × DBA/2J mouse combination) pregnancy loss. RNase protection analysis demonstrated an increase in TNF-α mRNA expression in the placentae of mice with pregnancy loss compared with that in control mice. TNF-α messages were localized to the uterine epithelium and stroma as well as the giant and spongiotrophoblast cells of the placenta. The intensity of the hybridization signal in placentae of mice with pregnancy loss was substantially higher than that in control mice. The up-regulation of TNF-α mRNA was accompanied by an increase in the expression of TNF-α receptor I mRNA in the same cell populations. The elevation of TNF-α production was also demonstrated at the protein level. Western blot analysis showed an increased level of the 18- and 26-kDa TNF-α protein species in the uteroplacental unit of mice with pregnancy loss. Immunostaining revealed TNF-α-positive leukocytes located in the uterus and placenta. Finally, we found that immunization of mice with cyclophosphamide-induced pregnancy loss while decreasing the resorption rate in these females resulted in a decline in TNF-α expression at the fetomaternal interface. These data clearly suggest an involvement of TNF-α in pathways leading to both spontaneous and induced placental death.