The embryonic development of mammalian neural tube defects

Interaction between splotch (Sp) and curly tail (ct) mouse mutants in the embryonic development of neural tube defects

The mouse mutations splotch (Sp) and curly tail (ct) both produce spinal neural tube defects with closely similar morphology, but achieve this by different embryonic mechanisms. To determine whether the mutants may interact during development, we constructed mice carrying both mutations. Double heterozygotes exhibited tail defects in 10% of cases, although the single heterozygotes do not express this phenotype. Backcrosses of double heterozygotes to ct/ct produced offspring with an elevated incidence of neural tube defects, both spina bifida and tail defects, compared with a control backcross in which Sp was not involved. Use of the deletion allele Sp2H permitted embryos carrying a splotch mutation to be recognised by polymerase chain reaction assay. This experiment showed that only embryos carrying Sp2H develop spina bifida in the backcross with ct/ct, suggesting that the genotype Sp2H/+, ct/ct is usually lethal around the time of birth as a result of severe disturbance of neurulation. The interaction between Sp and ct was investigated further by examining embryos in the backcross for developmental markers of the Sp/Sp and ct/ct genotypes. Sp/Sp embryos characteristically lack neural crest derivatives, such as dorsal root ganglia, and die on day 13 of gestation. Double mutant embryos from the backcross did not exhibit either of these characteristics suggesting that homozygosity for ct does not cause Sp/+ embryos to develop as if they were of genotype Sp/Sp. The angle of ventral curvature of the posterior neuropore region is enhanced in affected ct/ct embryos whereas it was found to be reduced in Sp/Sp embryos compared with their normal littermates.(ABSTRACT TRUNCATED AT 250 WORDS)

Malformation of the lumbosacral spinal cord in a case of sacral agenesis

Agenesis of the sacrum is a rare anomaly that is associated with numerous visceral abnormalities, spinal cord malformation, and lower limb defects. A fatal case of sacral and lower lumbar agenesis in a 3-day-old female infant born at 38 weeks of gestation is reported. The extraneural malformations comprised an imperforate anus, a rectovaginal fistula, and musculoskeletal abnormalities, including several thoracocervical hemivertebrae and aplasia of the sacrum and the fourth and fifth lumbar vertebrae. The cervical and high thoracic spinal cord segments were normal. Disruption of secondary neurulation, possibly due to notochord dysfunction, was suggested by malformation of the ventral half of the lower thoracic spinal cord with relative preservation of the dorsal horns and, more caudally, by loss of all normal histological landmarks, including the central canal. Neither skeletal muscle nor myoblasts were found in muscle compartments that would normally have received motor innervation from the levels of the spinal cord from which anterior horn cells were absent, indicating parallel, segmental failure of myotomal differentiation in the caudal eminence.

Disruption of the csk gene, encoding a negative regulator of Src family tyrosine kinases, leads to neural tube defects and embryonic lethality in mice

All Src family non-receptor tyrosine kinases are negatively regulated by phosphorylation at a carboxy-terminal tyrosine. To analyze the significance of this regulation during development, we have generated mice deficient in Csk, a kinase that phosphorylates this tyrosine, by gene targeting in embryonic stem cells. Homozygous mutant embryos exhibit a complex phenotype that includes defects in the neural tube and die between day 9 and day 10 of gestation. Cells derived from these embryos exhibit an order of magnitude increase in activity of Src and the related Fyn kinase. Phosphorylation at the carboxy-terminal tyrosine of Src was reduced but not eliminated and was accompanied by increased phosphorylation at another key tyrosine residue. These results demonstrate that Src family kinase activity is critically dependent on phosphorylation by Csk and suggest that the regulation of kinase activity may be essential during embryogenesis.

Postulated mechanisms underlying the development of neural tube defects. Insights from in vitro and in vivo studies

In recent years, use of animal models has resulted in acquisition of a significant amount of new information regarding normal and abnormal neural tube development. Studies of mutant and of teratogen-exposed mice are complementary, with each providing insights that promise to advance our understanding of the other. Analysis of teratogen-exposed embryos is best suited for identifying susceptible developmental stages and vulnerable populations. Advances in molecular genetics, with the ability to identify gene products, their cell/tissue location, and, potentially, to understand their function, will make naturally occurring as well as man-made mutants invaluable for understanding the heterogeneous mechanisms that underly NTDs.

Effects of altered maternal metabolism during gastrulation and neurulation stages of embryogenesis

In summary, many congenital malformations are produced during gastrulation and neurulation stages of embryogenesis at a time when no definitive chorioallantoic placenta has been established. In rodents, altered maternal metabolism may have a direct impact on the embryo or an indirect impact via disruption of the nutritive function of the visceral yolk sac. If similar mechanisms operate in human embryos, these factors probably alter functions of the trophoblastic shell. In any case, it is crucial to remember that the metabolic status of the embryo is rapidly changing and during early stages of organogenesis may respond to alterations in nutrients quite differently during the first four weeks of gestation than at later stages of organogenesis and the fetal period.

Intrinsic and extrinsic factors in the mechanism of neurulation: effect of curvature of the body axis on closure of the posterior neuropore

Neurulation has been suggested to involve both factors intrinsic and extrinsic to the neuroepithelium. In the curly tail (ct) mutant mouse embryo, final closure of the posterior neuropore is delayed to varying extents resulting in neural tube defects. Evidence was presented recently (Brook et al., 1991 Development 113, 671–678) to suggest that enhanced ventral curvature of the caudal region is responsible for the neurulation defect, which probably originates from an abnormally reduced rate of cell proliferation affecting the hindgut endoderm and notochord, but not the neuroepithelium (Copp et al., 1988, Development 104, 285–295). This axial curvature probably generates a mechanical stress on the posterior neuropore, opposing normal closure. We predicted, therefore, that the ct/ct posterior neuropore should be capable of normal closure if the neuropore should be capable of normal closure if the neuroepithelium is isolated from its adjacent tissues. This prediction was tested by in vitro culture of ct/ct posterior neuropore regions, isolated by a cut caudal to the 5th from last somite. In experimental explants, the neuroepithelium of the posterior neuropore, together with the contiguous portion of the neural tube, were separated mechanically from all adjacent non-neural tissues. The posterior neuropore closed in these explants at a similar rate to isolated posterior neuropore regions of non-mutant embryos. By contrast, control ct/ct explants, in which the caudal region was isolated but the neuroepithelium was left attached to adjacent tissues, showed delayed neurulation. To examine further the idea that axial curvature may be a general mechanism regulating neurulation, we cultured chick embryos on curved substrata in vitro. Slight curvature of the body axis (maximally 1 degree per mm axial length), of either concave or convex nature, resulted in delay of posterior neuropore closure in the chick embryo. Both incidence and extent of closure delay correlated with the degree of curvature that was imposed. We propose that during normal embryogenesis the rate of neurulation is related to the angle of axial curvature, such that experimental alterations in curvature will have differing effects (either enhancement or delay of closure) depending on the angle of curvature at which neurulation normally occurs in a given species, or at a given level of the body axis.

A mutation within intron 3 of the Pax-3 gene produces aberrantly spliced mRNA transcripts in the splotch (Sp) mouse mutant

The splotch (Sp) mouse mutant displays defects in neural tube closure in the form of exencephaly and spina bifida. Recently, mutations in the Pax-3 gene have been described in the radiation-induced Spr and Sp2H alleles. This led us to examine the integrity of the Pax-3 gene and its cellular mRNA transcript in the original, spontaneously arising Sp allele. A complex mutation in the Pax-3 gene including an A-->T transversion at the invariant 3' AG splice acceptor of intron 3 was identified in the Sp/Sp mutant. This genomic mutation abrogates the normal splicing of intron 3, resulting in the generation of four aberrantly spliced mRNA transcripts. Two of these Pax-3 transcripts make use of cryptic 3' splice sites within the downstream exon, generating small deletions which disrupt the reading frame of the transcripts. A third aberrant splicing event results in the deletion of exon 4, while a fourth retains intron 3. These aberrantly spliced mRNA transcripts are not expected to result in functional Pax-3 proteins and are thus responsible for the phenotype observed in the Sp mouse mutant.

Occurrence of embryotoxicity in mouse embryos following in utero exposure to 2'-deoxycoformycin (pentostatin)

Previous investigations had shown that i.p. injection of 2'-deoxycoformycin (dCF; pentostatin; 5 mg/kg) on either E7 or E8 into pregnant mice results in a 61-81% resorption rate at E17. The incidence of visible gross malformations among the surviving conceptuses was exceptionally low (3%) at the time of necropsy on E17 and was unrelated to dCF dose (Knudsen et al., Teratology, 40:5-626, '89; Teratology, 45:91-103, '92). These findings demonstrated the embryotoxicity of dCF but provided no clues as to the site(s) of dCF action. To define the lesion site(s), we have now examined embryos at 72 h (E10), 96 h (E11), and 120 h (E12) following administration of a highly embryotoxic dose of 5 mg dCF/kg to dams on E7. Deoxycoformycin caused multiple abnormalities and growth retardation, and the temporal sequence between maximal abnormal embryo incidence and resorption frequency was established. The quantitative data show that the maximal occurrence of abnormal embryos on E10 (71%) was followed by a maximal resorption rate on E12 (78%). There was a strong correlation (r = -0.82; P < 0.05) between the rapid decline of percent abnormal embryos over E10-E12 and the simultaneous increase in resorption rate, with linear regression analysis showing nearly equal but opposite slopes (-31.2% vs. +35.8% per gestational day, respectively). This suggests that one or more of the abnormalities seen at E10 is associated with the death and resorption of the embryo at E12. The dCF treatment perturbed a wide spectrum of developmental events, including neural tube closure, craniofacial and limb development, turning of the embryo, and growth retardation. None of the individual abnormalities, however, can quantitatively account for the high percentage of dead and resorbed embryos. Therefore, the specific cause of dCF-induced embryolethality is not clear. There is evidence both for direct dCF toxicity at specific embryonic sites as well as for a generalized retardation in the rate of development.

Mice homozygous for a targeted disruption of the proto-oncogene int-2 have developmental defects in the tail and inner ear

We derived mice that carry a targeted insertion of a neor gene in the int-2 (Fgf-3) proto-oncogene coding sequences. The mutation was found to be recessive and mice that were homozygous for the insertion did not often survive to adulthood. The mutant mice had defects in the development of the tail and inner ear that could be correlated with disruption of int-2 expression in the posterior primitive streak and hindbrain or otic vesicle. While the tail phenotype was 100% penetrant, we found that the inner ear phenotype had reduced penetrance and variable expressivity. The variable expressivity could not be attributed to variability in the genetic background of the mutant allele or to leaky expression from the mutant allele. Thus, we conclude that even in a uniform genetic background, stochastic variation in the expression of a developmental circuit can result in dramatic differences in phenotypic consequences.

Exogenous transferrin is taken up and localized by the neurulation-stage mouse embryo in vitro

We have screened neurulation-stage mouse embryos for regional differences in protein distribution, by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The screen has revealed an 83-kD protein (pI 6.8) that is present in embryo regions where neurulation is in progress but not in regions where neurulation is complete. The 83-kD protein is not synthesized in the neurulation-stage embryo or in the yolk sac, but is taken up from the culture serum in vitro and, probably, from the maternal serum in utero. The 83-kD protein has been identified as transferrin on the basis of its electrophoretic migration and recognition on Western blots by an antitransferrin antibody. Culture of embryos in serum containing 125I-transferrin, followed by autoradiography of embryo sections, shows that transferrin is taken up and localized in the gut beneath the closing neural folds at several levels of the body axis in 8.5- and 9.5-day embryos. In situ hybridization studies show that the transferrin receptor mRNA is expressed in all cells of the 9.5-day embryo, including the gut endoderm. These findings are consistent with a role for transferrin in development of the gut and perhaps, indirectly, in completion of neurulation during early mouse embryogenesis.

Spina bifida aperta induced by valproic acid and by all-trans-retinoic acid in the mouse: distinct differences in morphology and periods of sensitivity

The antiepileptic drug valproic acid (VPA) has been implicated as a human teratogen causing spina bifida aperta. Recently, we developed a mouse model inducing spina bifida aperta with VPA. To elucidate the pathogenesis of VPA-induced spina bifida aperta we now investigated the anatomy and histology of this defect in the mouse. The morphology of spina bifida aperta induced by all-trans-retinoic acid (RA) was used for comparison. Various doses of VPA and RA were administered at different times to determine the periods of sensitivity for inducing spina bifida aperta with these drugs. Each administration regimen consisted of three doses applied at intervals of 6 hr. RA induced spina bifida aperta during an earlier developmental period (day 8 of gestation) than VPA (day 9 of gestation). The most effective regimens for induction of spina bifida aperta in mice were injections of 3 x 500 mg VPA-Na/kg body weight (b.w.) intraperitoneally on day 9 of gestation at 0, 6, and 12 hr; RA (12.5 mg/kg b.w.) was given orally on day 8 of gestation at 12 and 18 hr, day 9 at 0 hr. VPA did not induce spina bifida aperta on day 8 of gestation and RA did not induce this effect on day 9 of gestation. Histological studies of day 18 fetuses carrying spina bifida aperta were performed. The spina bifida aperta induced by VPA shows a disorganized and necrotic spinal cord. In the vertebral canal were observed cell debris, blood cells, capillaries, macrophages, and rests of meninges. These results indicate that the spinal cord is almost destroyed at the affected section. In contrast, the spina bifida aperta induced by RA demonstrates a spinal cord organized in the gray and white matter, the dorsal and ventral horn. But the neural canal does not exist, only a layer of ependymal cells lies on the surface of the spinal cord. Our results indicate that the morphology of spina bifida aperta induced by VPA differed distinctly from that induced by RA in the mouse fetus. Moreover VPA produced a spina bifida aperta with a specific morphology. Also the period of sensitivity for induction of this lesion differed and occurred earlier for RA than for VPA. VPA and RA may possibly induce spina bifida aperta via different mechanisms in the mouse.

Inositol deficiency increases the susceptibility to neural tube defects of genetically predisposed (curly tail) mouse embryos in vitro

Curly tail (ct/ct) mouse embryos, which have a genetic predisposition for neural tube defects (NTD), were grown in culture from the 2-5 somite stage, before the initiation of neurulation, up to the 22-24 somite stage, when closure of the anterior neural tube is normally complete. The embryos were cultured in whole rat serum or in extensively dialysed serum supplemented with glucose, amino acids, and vitamins, with inositol omitted or added at concentrations of 2, 10, 20, and 50 mg/l. Two strains were used as controls; CBA mice, which are related to curly tails, and an unrelated PO stock. It was found that ct/ct embryos were particularly sensitive to inositol deficiency; both they and the CBA embryos showed a similar high incidence of cranial NTD after culture in inositol deficient medium (12/17 and 11/18, respectively). Furthermore, the lowest dose of inositol had no effect on the frequency of head defects in ct/ct mice, though it halved the incidence in CBA embryos. With higher inositol concentrations, the majority of ct/ct embryos completed head closure normally, and their development was generally similar to that obtained in whole serum. PO embryos showed a lower proportion (5/19) of cranial NTD in the inositol deficient medium than the other two strains, and this was further reduced by even the lowest inositol dose.

Maternal methionine supplementation promotes the remediation of axial defects in Axd mouse neural tube mutants

The Axd (axial defects) mouse model system (Essien et al., Teratology 42:183-194, '90) is characterized by a dominant mutation which causes posterior open neural tube defects (NTD) and a variety of tail anomalies (curly tails, or CT). Repeated backcrosses to BALB/cByJ mice have resulted in a 50% increase in Axd penetrance among neonates of heterozygous matings and loss of a correlation with maternal tail phenotype. Analysis of D12-D18 embryos from Axd/+ x Axd/+ matings indicates that soft tissues can superficially heal over some lesions from open NTD and that some curly tails can straighten (macroscopically) as gestation proceeds. Similarly, in embryos of Axd/+ x BALB crosses, there is remediation of approximately 33% of the tail flexion defects by birth. Numerous studies show that maternal nutritional status can affect the development of the neural tube and related axial structures. One nutrient of special interest is methionine, which is required for neurulation in cultured rat embryos (Coelho et al., J. Nutr. 119:1716, '89). Thus, the major question addressed by this study was whether supplemental methionine administered to Axd/+ dams crossed to Axd/+ males would alter the prenatal expression of the gene. When given IP (70 mg/kg) on D8 and D9, methionine resulted in a 41% reduction (from 29% to 17%) in the incidence of NTD in D 14 embryos (P less than 0.01).

Deceleration and acceleration in the rate of posterior neuropore closure during neurulation in the curly tail (ct) mouse embryo

Curly tail (ct) is a mouse mutant producing spinal neural tube defects as a result of delayed closure of the posterior neuropore (PNP). The purpose of the present study was to determine in ct/ct embryos the time of onset of the delay in PNP closure, and the pattern of this closure, as well as to study the possibility that reopening of the neural tube occurs. Normal spinal neurulation was studied in non-mutant Swiss (Sw) embryos. In the latter, the average PNP length diminished steadily between the 7- and 25-somite stages, and then decreased more rapidly, indicating an acceleration of closure rate, until the 30- to 32-somite stage, when all PNPs closed. PNP width decreased steadily between the stages of 7 and 30 somites. In ct/ct embryos the average PNP length showed a slight increase between the stage of 23 to 28 somites, indicating a temporary deceleration of closure rate, and the range of PNP sizes increased markedly. This was followed by a decrease in PNP length until the 37-somite stage, indicating an acceleration of closure rate. From the stage of 32 somites onwards, the proportion of embryos with closed PNPs gradually increased to 90%. The population of ct/ct embryos was subdivided. Embryos with large PNPs showed a marked deceleration of closure rate during a period of 11 somite stages, followed by a brief but very high acceleration of closure rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Splotch (Sp2H), a mutation affecting development of the mouse neural tube, shows a deletion within the paired homeodomain of Pax-3

The molecular basis of the mouse mutation splotch (Sp), which is associated with spina bifida and exencephaly, was analyzed at three of its alleles, Sp, Sp2H, and Spr. We mapped the paired box gene Pax-3 within the Inha to Akp3 interval, near or at the Sp locus on chromosome 1, and found Pax-3 to be deleted in heterozygous Spr/+ mice. Analysis of genomic DNA and cDNA clones constructed from Sp2H/Sp2H embryos identified a deletion of 32 nucleotides in the Pax-3 mRNA transcript and gene. This deletion maps within the paired homeodomain of PAX-3 and is predicted to create a truncated protein as a result of a newly created termination codon at the deletion breakpoint. Our study provides evidence for a causal link between deletion of the paired homeodomain of Pax-3 and the Sp2H mutation, and infers that Pax-3 plays a key role in normal neural development.

Valproic acid-induced neural tube defects in mouse and human: aspects of chirality, alternative drug development, pharmacokinetics and possible mechanisms

Administration of the antiepileptic drug valproic acid (VPA) during early pregnancy can result in a 1-2% incidence of spina bifida aperta, a closure defect of the posterior neural tube in the human. The predominant defect produced by VPA in the mouse is exencephaly, a closure defect of the anterior neural tube. Recent experiments demonstrate that an appropriate dosing regimen (consecutive doses of VPA on day 9 of gestation) can also result in a low incidence of spina bifida aperta, and a high incidence of spina bifida occulta in the mouse as a potential animal model. Relatively high doses and concentrations of VPA are needed in the mouse to produce neural tube defects, the human appears to be more sensitive in this regard. Maximal concentrations and not AUC (area under the concentration-time curve) values correlate with the incidence of neural tube defects in the mouse which could in part be explained by saturation of plasma protein binding, increased free drug available for placental transfer and the embryonic neuroepithelium acting as a "deep compartment". It is likely that the parent drug and not a metabolite is the proximate teratogen. Structure-activity relationships show a strict structural requirement for high teratogenic potency: the molecule must contain an alpha-hydrogen atom, a carboxyl function, branching on carbon atom 2 with two chains containing 3 carbon atoms each for maximum activity. If these two carbon chains are different, then enantiomers are present such as the R- and S-enantiomers of 2-n-propyl-4-pentenoic acid (4-en-VPA), 2-n-propyl-4-pentynoic acid (4-yn-VPA) and 2-ethylhexanoic acid. These enantiomers were synthesized and shown to be significantly different in regard to teratogenic potency. Pharmacokinetic studies indicate that both enantiomers of each compound reach the embryo to the same degree. Therefore, the intrinsic teratogenic activity of the enantiomers differ, suggesting a stereoselective interaction between the drugs and a chiral structure within the embryo, is involved in the mechanism of action. In sharp contrast to the teratogenic effect, the anticonvulsant activity and neurotoxicity of this compound class show broad structural specificity, opening the possibility for development of novel antiepileptic agents with low teratogenic potency such as 2-n-propyl-2-pentenoic acid (2-en-VPA). The molecular mechanism of the teratogenicity of VPA is quite unknown; of the several hypothesis suggested, the interaction of VPA with embryonic folate metabolism is discussed here.

Disruption of the Hox-1.6 homeobox gene results in defects in a region corresponding to its rostral domain of expression

The Hox-1.6 gene disrupted in embryonic stem cells by homologous recombination was introduced into the mouse germline. Heterozygous mice were normal, but homozygous mice died at birth from anoxia and had numerous defects that were centered at the level of rhombomeres 4 to 7 and included delayed hindbrain neural tube closure, absence of certain cranial nerves and ganglia, and malformed inner ears and bones of the skull. Thus, Hox-1.6 is involved in regional specification along the rostrocaudal axis, but only in its most rostral domain of expression. Hox-1.6 appears to specify neurogenic neural crest cells prior to specification of mesenchymal neural crest cells by Hox-1.5. Thus, within the same region of the presumptive hindbrain, two HOX-1 genes are involved in the patterning of two different populations of neural crest cells. The implication of these results for the function of the Hox network during mouse embryogenesis is discussed.