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EphrinA-EphA receptor interactions in mouse spinal neurulation: implications for neural fold fusion.

The molecular basis of neural fold adhesion and fusion is a poorly understood aspect of neurulation. Cell surface glycosyl phosphatidylinositol (GPI)-anchored proteins have been implicated in neural fold adhesion, with ephrinAs particularly attractive candidates in view of the cranial neural tube defects observed in mice lacking ephrinA5 or the EphA7 receptor. Here, we demonstrate that ephrinsA1, A3 and A4, as well as several EphA receptors, are expressed in the closing mouse spinal neural tube. Most ephrinAs and EphA receptors were found to be expressed in multiple tissues in the caudal region, whereas EphA2 receptor was expressed specifically at the apices of the neural folds just prior to onset of neural tube fusion. Using mouse whole embryo culture, we found that cleavage of GPI-anchored molecules from the embryonic cell surface resulted in delay of spinal neural tube closure. Injection of EphA1 and EphA3 fusion proteins intra-amniotically into cultured embryos was used to specifically disrupt ephrinA-EphA receptor interactions, and led to inhibition of spinal neural tube closure, without adverse effects on growth or developmental progression. These treatments did not disturb neural plate bending or neural fold elevation, both of which are critical for spinal neural tube closure. Our findings demonstrate that ephrinA-EphA receptor interactions are required for closure of the mouse spinal neural tube, and support the hypothesis that ephrinA-EphA receptor interactions may participate in the molecular recognition events that culminate in adhesion and fusion of the tips of the neural folds during spinal neurulation.