Cellular mechanisms underlying Pax3-related neural tube defects and their prevention by folic acid

Neural tube defects (NTDs), including spina bifida and anencephaly, are among the most common birth defects worldwide, but their underlying genetic and cellular causes are not well understood. Some NTDs are preventable by supplemental folic acid. However, despite widespread use of folic acid supplements and implementation of food fortification in many countries, the protective mechanism is unclear. Pax3 mutant (splotch; Sp2H ) mice provide a model in which NTDs are preventable by folic acid and exacerbated by maternal folate deficiency. Here, we found that cell proliferation was diminished in the dorsal neuroepithelium of mutant embryos, corresponding to the region of abolished Pax3 function. This was accompanied by premature neuronal differentiation in the prospective midbrain. Contrary to previous reports, we did not find evidence that increased apoptosis could underlie failed neural tube closure in Pax3 mutant embryos, nor that inhibition of apoptosis could prevent NTDs. These findings suggest that Pax3 functions to maintain the neuroepithelium in a proliferative, undifferentiated state, allowing neurulation to proceed. NTDs in Pax3 mutants were not associated with abnormal abundance of specific folates and were not prevented by formate, a one-carbon donor to folate metabolism. Supplemental folic acid restored proliferation in the cranial neuroepithelium. This effect was mediated by enhanced progression of the cell cycle from S to G2 phase, specifically in the Pax3 mutant dorsal neuroepithelium. We propose that the cell-cycle-promoting effect of folic acid compensates for the loss of Pax3 and thereby prevents cranial NTDs.

Integrity of the methylation cycle is essential for mammalian neural tube closure

BACKGROUND

Closure of the cranial neural tube during embryogenesis is a crucial process in development of the brain. Failure of this event results in the severe neural tube defect (NTD) exencephaly, the developmental forerunner of anencephaly.

METHODS

The requirement for methylation cycle function in cranial neural tube closure was tested by treatment of cultured mouse embryos with cycloleucine or ethionine, inhibitors of methionine adenosyl transferase. Embryonic phenotypes were investigated by histological analysis, and immunostaining was performed for markers of proliferation and apoptosis. Methylation cycle intermediates s-adenosylmethionine and s-adenosylhomocysteine were also quantitated by tandem mass spectrometry.

RESULTS

Ethionine and cycloleucine treatments significantly reduced the ratio of abundance of s-adenosylmethionine to s-adenosylhomocysteine and are, therefore, predicted to suppress the methylation cycle. Exposure to these inhibitors during the period of cranial neurulation caused a high incidence of exencephaly, in the absence of generalized toxicity, growth retardation, or developmental delay. Reduced neuroepithelial thickness and reduced density of cranial mesenchyme were detected in ethionine-treated but not cycloleucine-treated embryos that developed exencephaly. Reduced mesenchymal density is a potential cause of ethionine-induced exencephaly, although we could not detect a causative alteration in proliferation or apoptosis prior to failure of neural tube closure.

CONCLUSIONS

Adequate functioning of the methylation cycle is essential for cranial neural tube closure in the mouse, suggesting that suppression of the methylation cycle could also increase the risk of human NTDs. We hypothesize that inhibition of the methylation cycle causes NTDs due to disruption of crucial reactions involving methylation of DNA, proteins or other biomolecules.

Excess methionine suppresses the methylation cycle and inhibits neural tube closure in mouse embryos

Suppression of one-carbon metabolism or insufficient methionine intake are suggested to increase risk of neural tube defects (NTD). Here, exogenous methionine unexpectedly caused frequent NTD in cultured mouse embryos. NTD were associated with reduced cranial mesenchyme cell density, which may result from a preceding reduction in proliferation. The abundance ratio of S-adenosylmethionine to S-adenosylhomocysteine was also decreased in treated embryos, suggesting methylation reactions may be suppressed. Such an effect is potentially causative as NTD were also observed when DNA methylation was specifically inhibited. Thus, reduced cranial mesenchyme density and impairment of critical methylation reactions may contribute to development of methionine-induced NTD.

Specific isoforms of protein kinase C are essential for prevention of folate-resistant neural tube defects by inositol

A proportion of neural tube defects (NTDs) can be prevented by maternal folic acid supplementation, although some cases are unresponsive. The curly tail mutant mouse provides a model of folate-resistant NTDs, in which defects can be prevented by inositol therapy in early pregnancy. Hence, inositol represents a possible novel adjunct therapy to prevent human NTDs. The present study investigated the molecular mechanism by which inositol prevents mouse NTDs. Activation of protein kinase C (PKC) is known to be essential, and we examined neurulation-stage embryos for PKC expression and applied PKC inhibitors to curly tail embryos developing in culture. Although all known PKC isoforms were detected in the closing neural tube, use of chemical PKC inhibitors identified a particular requirement for 'conventional' PKC isoforms. Peptide inhibitors offer selective inhibition of individual PKCs, and we demonstrated isoform-specific inhibition of PKC in embryonic cell cultures. Application of peptide inhibitors to neurulation-stage embryos revealed an absolute dependence on the activity of PKCbetaI and gamma for prevention of NTDs by inositol, and partial dependence on PKCzeta, whereas other PKCs (alpha, betaII delta, and epsilon) were dispensable. To investigate the cellular action of inositol and PKCs in NTD prevention, we examined cell proliferation in curly tail embryos. Defective proliferation of hindgut cells is a key component of the pathogenic sequence leading to NTDs in curly tail. Hindgut cell proliferation was stimulated specifically by inositol, an effect that required activation of PKCbetaI. Our findings reveal an essential role of specific PKC isoforms in mediating the prevention of mouse NTDs by inositol.