The role of transcriptional repression in Shh signalling and patterning of the ventral neural tube

Abstract

From flies to humans, members of the Hedgehog (Hh) family of secreted signalling molecules play a vital role in determining cell fate and patterning during embryonic development. In vertebrates, Sonic Hedgehog (Shh) act in a graded manner to establish distinct neural progenitor populations at different concentration thresholds in the ventral neural tube. This is achieved by regulating the expression pattern of progenitor homeodomain (HID) proteins. The expression profiles of these HID proteins define five progenitor domains in the ventral part of the neural tube that will each generate a distinct subtype of neuron.

We set out to resolve the mechanism by which HID proteins mediate their patterning activities and found that most HID proteins involved in ventral neuronal patterning possess a conserved engrailed homology (eh1) motif related to the core region of the Engrailed (En) repressor domain. This domain mediates interactions of progenitor HID proteins with Groucho/TLE (Gro/TLE) corepressors and renders a repressor function to them that is essential for the neural patterning activity of these HID proteins in vivo. Taken together, our observations indicate that the pattern of neuronal generation in the ventral neural tube is achieved through the spatially controlled repression of transcriptional repressors - a derepression model of neuronal fate specification.

Members of the Ci/Gli family of transcription factors have been implicated as being the down stream mediators of the Hh signal. In vertebrates three Gli genes (Gli1, Gli2 and GIB) have been found that are all expressed in the neural tube. We examined the role of Gli proteins in patterning of the ventral neural tube by analysing mice lacking GIi3. Our data identify a role for GIB in patterning of the intermediate region of the spinal cord and also show that a repressor form of GIB is able to fully substitute for the full length gene in all aspects of dorsal-ventral patterning.

Furthermore, analysis of Shh/GIi3 compound mutant mice substantiates the idea that ventral patterning may involve a mechanism independent, or parallel, to graded Shh signalling. In summary, our observations raise the possibility that Gli proteins act by integrating Shh signals and other sources of positional information, to control patterning of the ventral neural tube.

The pattern of distinct neuronal cell types in the ventral neural tube is generally believed to reflect a concentration gradient of Shh. However, we provide evidence that this patterning also relies on changes in the competence of cells to respond to and interpret Shh signalling over time. We show that these changes in competence are directly controlled by the temporal expression of HID proteins that alters the responsiveness of cells to Shh.

Furthermore, these competence changes are necessary for the correct patterning of the three ventral-most cell types of the neural tube and take place independent of the Shh gradient. The spatial differences in the generation of these cell types involve a "non-neuronal-to-neuronal" shift in progenitor potential over time. We propose that temporal alterations of competence is an integral component of the morphogen activity of Shh, and that competence-based patterning requires active Shh signalling but not necessarily a graded distribution of the Shh protein.