A homeodomain feedback circuit underlies step-function interpretation of a Shh morphogen gradient during ventral neural patterning

Development, Dec 2010

Madelen Lek, José M. Dias, Ulrika Marklund, Christopher W. Uhde, Sanja Kurdija, Qiubo Lei, Lori Sussel, John L. Rubenstein, Michael P. Matise, Hans-Henning Arnold, et al.

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A homeodomain feedback circuit underlies step-function interpretation of a Shh morphogen gradient during ventral neural patterning

Madelen Lek 0 Jos M. Dias Ulrika Marklund Christopher W. Uhde Sanja Kurdija Qiubo Lei LoriSussel John L. Rubenstein Michael P. Matise Hans-Henning Arnold Thomas M. Jessell JohanEricson 0 Present address: UBE , Nobels vag15A , Karolinska Institutet , S-17177 Stockholm , Sweden SUMMARY The deployment of morphogen gradients is a core strategy to establish cell diversity in developing tissues, but little is known about how small differences in the concentration of extracellular signals are translated into robust patterning output in responding cells. We have examined the activity of homeodomain proteins, which are presumed to operate downstream of graded Shh signaling in neural patterning, and describe a feedback circuit between the Shh pathway and homeodomain transcription factors that establishes non-graded regulation of Shh signaling activity. Nkx2 proteins intrinsically strengthen Shh responses in a feed-forward amplification and are required for ventral floor plate and p3 progenitor fates. Conversely, Pax6 has an opposing function to antagonize Shh signaling, which provides intrinsic resistance to Shh responses and is important to constrain the inductive capacity of the Shh gradient over time. Our data further suggest that patterning of floor plate cells and p3 progenitors is gated by a temporal switch in neuronal potential, rather than by different Shh concentrations. These data establish that dynamic, non-graded changes in responding cells are essential for Shh morphogen interpretation, and provide a rationale to explain mechanistically the phenomenon of cellular memory of morphogen exposure. INTRODUCTION A general strategy to establish cellular diversity in developing organisms is the deployment of morphogen gradients that signal at long range from localized sources to impart positional information to surrounding cells (Lander, 2007). A well-studied example in vertebrate development is the secreted protein Sonic hedgehog (Shh), which is responsible for conveying such information to cells of the ventral neural tube (Jessell, 2000) and to other tissues such as the limb bud (Bastida and Ros, 2008). In the neural tube, this activity is manifested in the patterned generation of distinct neuronal subtypes at stereotypic positions in response to graded Shh signaling by ventral midline cells. A key role for Shh in this process is to regulate expression of patterning transcription factors, but the mechanisms by which information is translated from the extracellular Shh gradient into a precise and robust transcriptional output remain poorly understood. Models of graded Shh signaling in the neural tube (Roelink et al., 1995; Ericson et al., 1997; Dessaud et al., 2007) posit that a long-range extrinsic spatiotemporal gradient of ligand emanates first from the notochord and then from the floor plate (FP). The Shh signal is received by neural progenitors and transduced by the Patched/Smoothened (Ptc1/Smo)-receptor complex, ultimately regulating activity of the executors of the Shh pathway, zinc-finger domain-containing transcription factors of the Gli family. In the absence of Shh, Gli2 and Gli3 are processed to transcriptional repressors (GliR), whereas both are stabilized in their activator forms (GliA) upon exposure to Shh ligand (reviewed by Dessaud et al., 2008). Gli proteins regulate the patterned expression of homeodomain (HD) and basic helix-loop-helix (bHLH) transcription factors in neural progenitors, and the cross-repressive interactions between these proteins are important for the establishment and maintenance of ventral progenitor domains expressing distinctive molecular profiles (Briscoe et al., 2000; Muhr et al., 2001). These transcription factors provide positional value to cells and hence determine the identity of neuronal subtypes generated from distinct progenitor populations (Dessaud et al., 2008). Evidence for graded activity of Shh originates primarily from ex vivo experiments, in which incremental two- to threefold increases in the ambient Shh concentration (Roelink et al., 1995; Ericson et al., 1997) or in the duration of exposure to Shh (Dessaud et al., 2007) result in induction of cell types that are characteristic of progressively more ventral regions of the neural tube. Shh signaling acts at long range in vivo (Briscoe et al., 2001) and a graded distribution of the Shh ligand has been demonstrated (Chamberlain et al., 2008). Together with genetic analyses of Gli genes (Ding et al., 1998; Matise et al., 1998; Persson et al., 2002) and gain-offunction studies in chick (Stamataki et al., 2005), these data are consistent with a model in which the extracellular Shh gradient is translated into an intracellular Gli activator-to-repressor gradient along the ventral-to-dorsal axis of the neural tube (Dessaud et al., 2008). Consistent with this view, ventral subtype identities are lost in mice that lack Shh, as well as in Smo mutants that are unable to transduce the Shh signal (Chiang et al., 1 (...truncated)


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Madelen Lek, José M. Dias, Ulrika Marklund, Christopher W. Uhde, Sanja Kurdija, Qiubo Lei, Lori Sussel, John L. Rubenstein, Michael P. Matise, Hans-Henning Arnold, Thomas M. Jessell, Johan Ericson. A homeodomain feedback circuit underlies step-function interpretation of a Shh morphogen gradient during ventral neural patterning, Development, 2010, pp. 4051-4060, 137/23, DOI: 10.1242/dev.054288