Cross-repressive Interactions Between Nkx2.2and Sp8/Sp9Specify The Identity Of P3and PMM Progenitors In The Spinal Cord

During the early development of vertebrate spinal cord, progenitor cells which are located in the ventricular zone (VZ) of the neural tube,pattern into multiple domains along the dorsoventral (DV) axis of the neural tube. Each progenitor domain can be identified by combinatorial expression of distinct transcription factors and generates particular types of neurons. Cross-repressive interactions between transcription factors play important roles in patterning neural tube. p3and pMN domains are the two ventral most neuronal progenitor domains, give rise to V3interneurons and motor neurons (MNs) respectively. Nkx2.2expressed in p3domain and Sp8expressed from pMN to ventral dP5domain. The ventral limit of Sp8expression is complementary to the dorsal limit of Nkx2.2expression. While overexpressed in chick embryos, Sp8and Nkx2.2exhibit mutual repressive interactions. Previous studies reveal that Nkx2.2overexpression induces ectopic V3interneurons throughout the DV axis, we found induction of ectopic MNs by Sp8overexpression is confined to the p3domain. The activator form of Sp8(VP16-Sp8c) mimics the ability of wild type Sp8proteins to repress Nkx2.2expression and to induce ectopic MNs within the p3domain. Overexpression an repressor form of Sp8(EnR-Sp8c) failes in repressing Nkx2.2expression, instead greatly impaires the production of MNs from the normal pMN domain. These rusults indicate that Sp8functions as a transcription activator to pattern ventral spinal cord. In the Nkx2.2mutant embryos Sp8expression expands ventrally occupies most of the presumptive p3domain. However, in Sp8conditional knock out mouse embryos, neither dorsal expansion of Nkx2.2expression nor MN generation defects were observed. Sp9is closed related to Sp8, we found Sp9is also expressed in the ventral spinal cord. Misexpression of Sp9in the p3domain converts p3progenitors to MN progenitors, just the same as Sp8overexpression. It is quite possible that Sp8and Sp9could function redundantly in ventral spinal cord patterning. These results suggest that Nkx2.2and Sp8/Sp9establish the distinction in the identity of the p3and pMN domains through cross-repressive interactions.