Studies on physiological functions of ovalbumin and occurrence of neural tube defects due to deprivation of ovalbumin in chick embryo.

Ovalbumin has been classified as a homolog of serine protease inhibitor (serpin), although little is known of its function except that it may be a nutrient source. We have shown that, during embryonic development, native molecules of ovalbumin (N-ovalbumin) are incorporated into tissues without degradation while undergoing configurational changes into a heat-stable form called 'HS-ovalbumin.' Here, to investigate the indispensability of HS-ovalbumin, we utilized an in vitro culture method by which chick embryos isolated from eggs at Stage 7 (before the neural tube begins to close) could develop normally for up to 24h. When endogenous, remnant ovalbumin was blocked by adding anti-ovalbumin polyclonal IgG to the medium, the isolated embryos became irregular particularly in brain tissue, resulting in the so-called neural tube deficiency (NTD). The embryos with NTD showed reduced signals for apoptosis and cell proliferation. Moreover, IgG-treated embryos were rescued by adding a sufficient amount of HS-ovalbumin to the medium, attaining normal morphologies without NTD. HS-ovalbumin exhibited a higher rescue rate than N-ovalbumin and S-ovalbumin, the latter being another heat-stable counterpart that occurs artificially by heating N-ovalbumin in vitro. These results indicate that ovalbumin (particularly in the HS-form) is essential in embryonic organogenesis, including neural tube formation. present results show also that the ovalbumin-deprived embryos gave decreased in situ signals for TUNEL reagents and for PCNA/c-Myc proteins, which mirror changes in apoptosis and cell proliferation, respectively. Also the ovalbumin deprival affect the embryos on many gene expressions including differentiation-inducing factors related to morphogenesis. Expressions of FGF8 and slug have decreased markedly, whereas those of BMP4 and Pax3 have increased over 3-fold. However, whether each of these alterations is a cause or a result of morphogenetic aberrancy is unclear. Since ovalbumin belongs to the serpin superfamily, there is a possibility that it shows direct interactions with some crucial differentiation-inducing factors.