Comparative studies of CNS development

Integration of multiple signalling centre by transcription factors

In the last years, research as shed light on the mechanism by which a secreted molecule, released by a signalling centre, establish cell fate specification in a given tissue. However, the mechanism by which a given cell integrates the information received from a set of signals (coming for one or multiple signalling centres) is yet to be elucidated. We use the telencephalon to tackle this issue, assessing how progenitors define their identity in response to two signalling centres: a ventral Hh-secreted and a dorsal Wnt-secreted centres. We identified the transcription factor Foxg1 as an integrator of these two signals (Danesin et al., 2009) and are now identifying the direct targets of Foxg1 in this process.

BMP signalling and induction of telencephalic fat

Depletion of Wingless (Wnt) signalling is a major requirement for the induction of proencephalic fates, comprising telencephalon and eye field (Heisenberg et al., 2001, Houart et al., 2002). However, the molecular basis of the differential regionalisation of medial (eye field) vs. marginal (Telencephalon) fates cannot be explained by a simple unidirectional signalling gradient along the anterior – posterior axis (like the Wnt gradient). One putative candidate "segregator" is the Bone Morphogenic Protein (BMP) which is secreted in the non neural ectoderm surrounding the neural plate both laterally and anteriorly during gastrulation. BMP mutants exhibit a massive expansion of eye field marker expression at the expense of telencephalon. Our current working hypothesis is that BMP signalling is repressing eye field markers in the telencephalon and therefore contributes to pattern marginal and medial neural fates during early development of the anterior neural plate in zebrafish.

Diencephalic fate specification

The diencephalon arises from the posterior prosencephalon and forms the hypothalamus, the prethalamus, the thalamus, the epithalamus, and the pretectum. These brain structures are vital to a variety of body functions such as the regulation of the endocrine system or the relaying of senses and motor signals. A fate-mapping study in our lab (Staudt and Houart, 2007) has helped to reveal the neural plate origins of cells that give rise to these diencephalic territories. The study demonstrated that very little mixing occurs between distinct cell populations and that cells of the prospective prethalamus are functionally specified and irreplaceable as early as the late neural plate stage. In an effort to unravel the genetic components of this specification, we have isolated and RNA-profiled cells of the diencephalic anlage and compared their transcriptome to this of the presumptive midbrain. Currently, we are allocating gene expression profiles to the cell populations defined by fate-mapping. We are identifying areas of restricted pathway activities and are assessing gene function for specific candidates.

Initiation of Eye Patterning

Vision is essential to survival as it provides an accurate perception of the world surrounding us. This perception is obtained by the ability of the retina to measure levels of light and transfer this information to the brain. The retina is formed by evagination of a forebrain territory located just posterior to the telencephalon inside the neural plate. A correct patterning of the retina is essential for proper connection between eye and the visual centres in the brain and to the processing of visual information. We are using zebrafish as our model organism to study the role of Dickkopf1 (Dkk1), a negative regulator of Wnt signalling, in the regulation of the dorsal signalling centre in the eye. Using mutant and transgenic zebrafish lines, and using gain and loss of function techniques, transplants and small molecules inhibitors, we study the mechanism by which the diencephalon influences eye patterning, by establishment of its dorsal-ventral axis.

Signalling molecules and control of cell motility

Our lab has shown that the Wnt antagonist Dkk1 is controlling cell motility during gastrulation. More importantly, it does so independently of the Wnt/bcatenin pathway, through a novel interaction with Glypican 4 and the Wnt/PCP pathway (Caneparo et al., 2007). We are currently exploring the nature of the molecular interaction leading to Dkk1 modulation of motility. Understanding what is/are the direct interactor(s) of Dkk1 in this process is of great interest not only to understand regulation of motility in vivo but will also have impact in understanding metastatic processes.