A direct requirement for Hedgehog signaling for normal specification of all ventral progenitor domains in the presumptive mammalian spinal cord

Abstract

The hedgehog signaling pathway organizes the developing ventral neural tube by establishing distinct neural progenitor fates along the dorsoventral axis. Smoothened (Smo) is essential for all Hedgehog (Hh) signaling, and genetic inactivation of Smo cells autonomously blocks the ability of cells to transduce the Hh signal. Using a chimeric approach, we examined the behavior of Smo null mutant neural progenitor cells in the developing vertebrate spinal cord, and we show that direct Hh signaling is essential for the specification of all ventral progenitor populations. Further, Hh signaling extends into the dorsal half of the spinal cord including the intermediate Dbx expression domain. Surprisingly, in the absence of Sonic hedgehog (Shh), we observe the presence of a Smo-dependent Hh signaling activity operating in the ventral half of the spinal cord that most likely reflects Indian hedgehog (Ihh) signaling originating from the underlying gut endoderm. Comparative studies of Shh, Smo, and Gli3 single and compound mutants reveal that Hh signaling acts in part to specify neural cell identity by counteracting the repressive action of Gli3 on p0, p1, p2, and pMN formation. However, whereas these cell identities are restored in Gli3/Smo compound mutants, correct stratification of the rescued ventral cell types is lost. Thus, Hh signaling is essential for organizing ventral cell pattern, possibly through the control of differential cell affinities.

Regulation and function of Dbx genes in the zebrafish spinal cord

“…However, cell-autonomous loss of all Hedgehog signaling by removal of Smoothened function leads to a shift in position, but not complete absence, of Dbx genes (Wijgerde et al, 2002). This experiment suggests that Hedgehog acts to set the position of Dbx-positive progenitors, but is not absolutely required for expression of these genes.…”

“…The effects of Hedgehog and retinoic acid (RA) signaling in regulating Dbx1/2 expression have been studied in mouse and chick (Pierani et al, 1999;Briscoe et al, 2001;Wijgerde et al, 2002;Novitch et al, 2003). Hedgehog is required for patterning the ventral spinal cord by either activating or repressing target genes by means of the Gli transcription factor family (Jacob and Briscoe, 2003).…”

“…Hedgehog is required for patterning the ventral spinal cord by either activating or repressing target genes by means of the Gli transcription factor family (Jacob and Briscoe, 2003). Shh Ϫ/Ϫ mouse embryos at early stages of development exhibit ventrally expanded Dbx1 expression (Wijgerde et al, 2002), while later in development, Dbx1 and Dbx2 expression are localized to the ventral midline of the spinal cord (Pierani et al, 1999;Wijgerde et al, 2002). These studies indicated that the effect of reducing Shh activity changes during development of the spinal cord and that the ventral midline expression of Dbx1 and Dbx2 might be indirectly caused by the ventral expansion of more dorsal progenitor domains.…”

“…These studies indicated that the effect of reducing Shh activity changes during development of the spinal cord and that the ventral midline expression of Dbx1 and Dbx2 might be indirectly caused by the ventral expansion of more dorsal progenitor domains. Additionally, it has been suggested that low levels of Hedgehog induce Dbx expression in intermediate domains and high levels of Hedgehog repress Dbx more ventrally (Briscoe et al, 2001;Wijgerde et al, 2002). However, Hedgehog signaling does not appear to be absolutely required for Dbx expression, because cells lacking Smoothened function are still capable of expressing these genes, albeit in ectopic locations (Wijgerde et al, 2002).…”

“…Additionally, it has been suggested that low levels of Hedgehog induce Dbx expression in intermediate domains and high levels of Hedgehog repress Dbx more ventrally (Briscoe et al, 2001;Wijgerde et al, 2002). However, Hedgehog signaling does not appear to be absolutely required for Dbx expression, because cells lacking Smoothened function are still capable of expressing these genes, albeit in ectopic locations (Wijgerde et al, 2002). Indirect regulation of Dbx expression may occur by a secondary mechanism by which Class I and Class II homeodomain genes cross-repress each other to refine the borders between different domains.…”