The XDBX Gene is a Direct Target of ZIC1 in Early Neurula Stage

Abstract

In both vertebrates and invertebrates homeobox genes encode transcriptional regulators that act to specify spatial domains within the developing embryo. In the early vertebrate embryo, the development of the nervous system depends upon striking a balance between differentiating neurons and neural progenitors (Gershon et al., 2000). The eventual fate of these progenitors is in part linked to the timing of their differentiation as neuronal subtypes during development. The temporal regulation of neuronal differentiation and the maintenance of a pool of neural progenitors, therefore, insure that the diver- sity of neuronal cell fates is generated during nervous system development (Lu et al., 1992).
More than 20 mammalian Antennapedia class I homeobox genes are expressed in the developing central nervous system (CNS). Their expression patterns are spatially restricted along both the anteroposterior and dorsoventral axis. The spatial restrictions within the dorsoventral axis of the CNS are limited and often extend over many cell layers. These genes generally have different anterior boundaries of expression that are never anterior to the hindbrain (Lu et al., 1992).
The establishing of correct dorsal-ventral patterning in spinal cord development is a key process in producing a functional spinal cord. Discrete domains of spinal progenitors express a code of transcription factors, and then produce moto-, inter-, and sensory-neurons. Studies in a variety of vertebrate species have described the extrinsic and intrinsic cues necessary for establishing the progenitor domains. For example, dbx genes encode a family of homeodomain transcription factors that define an intermediate spinal progenitor domain (Lu et al., 1992). This gene family has multiple functions in spinal cord development. In Xenops laevis, Xdbx inhibits neurogenesis by regulating Xash3 expression at neural plate stages (Gershon et al., 2000), whereas in mouse and chick, studies have shown that dbx1-2 are necessary in spinal cord development for the production of V0-V1 interneurons (Pierani et al., 2001), radial glia, astro- cytes, and oligodendrocytes (Fogarty et al., 2005). The Xenopus embryo provides a useful system for studying the factors that maintain this balance as undifferenti- ated progenitors and differentiating neu- rons are maintained in a stereotypic, re- gionalized pattern within the neural plate (Hartenstein, 1993).