Centre de Biologie du Développement

Centre de Biologie du Développement
UMR5547: CNRS/ Université de Toulouse
Bat4R3, 118 route de Narbonne
31062, Toulouse, France


patrick.blader univ-tlse3.fr
Phone: +33 (0)5 61 55 67 34
Fax: +33 (0)5 61 55 65 07

Control of neuronal identity in the zebrafish

Our work concerns the three following axes of research, the first primarily historic and later two more recent, all of which deal with various aspects of neurogenesis using the zebrafish as a model.

1) Previously, we have studied regulatory element required for the expression of the zebrafish proneural gene neurogenin1 (neurog1) (Blader et al., 2004). We are now addressing how this bHLH transcription factor regulates its targets. Indeed, we have shown that Neurog1 regulates its target deltaA via three non-redundant E-boxes (Madelaine and Blader, 2011).

More recently, we have shown that, neurog1 and a second zebrafish Atonal genes neurod4, are redundantly required for development of both early-born olfactory neurons (EON) and later olfactory sensory neurons (OSN). We show that neurod4 expression is initially regulated by Neurog1 but recovers in the neurog1 mutant and is sufficient for the delayed development of OSN. In contrast, EON numbers are significantly reduced in neurog1 mutant embryos despite the recovery of neurod4 expression. Our results suggest that a shortened time window for EON development causes this reduction; EON leave the cell cycle later in neurog1 mutant embryos than in siblings but are all post-mitotic at the same stage as wild type embryos. Finally, we show that the expression of the EON specific gene robo2 is never detected in neurog1 mutant embryos. Failure of robo2 expression to recover correlates with defects in the fasciculation of neurog1 mutant olfactory axonal projections and the organisation of proto-glomerulii when projections arrive at the olfactory bulb that are comparable to those seen in robo2/astray mutant embryos. We conclude that the duration of proneural expression in EON progenitors is critical for correct development of the zebrafish olfactory system (Madelaine et al., 2011).

2) The zebrafish epithalamus is composed of the left and right habenular nuclei and the pineal complex, which itself consists of the medially positioned epiphysis and the left-sided parapineal (for review see Roussigné et al., 2012); the habenulae also display left-right (LR) asymmetries that include differences in gene expression, neuropil organization and connectivity patterns. LR asymmetry in the epithalamus is established via a stepwise sequence of interactions between the left presumptive habenula and the migrating parapineal. Studies have shown that handedness of these asymmetry are biased by unilateral Nodal signalling. We have shown a L/R asymmetry in the pool of neurog1+ habenular progenitors and that this asymmetry is compromised in the absence of Nodal signalling (Roussigné et al., 2009); more recent results from the group indicate that Nodal imposes this neurogenetic asymmetry via the transcription factor Pitx2c (Garric et al., in prep). In the absence of Nodal signalling, however, habenular neurogenesis is not absent suggesting that Nodal/Pitx2c influences a generic neurogenesis pathway. We have unpublished evidence that this pathway requires Sonic hedghog signalling, Pax6 and ultimately the redundant activities of Neurog1 and NeuroD4 (Halluin et al., in prep).

3) The zebrafish epiphysis is a small, light-sensitive vesicle in the epithalamus containing only two neural subtype, projection neurons or photoreceptors, and as such provides a simple system for understanding how different neural subtypes are specified. We have identified the Notch pathway as an actor in making the choice between these two cell fates (Cau et al., 2008). Our results from this study suggest that at least two distinct signals are required for epiphysial neural fate specification: one, involving Notch, for the inhibition of projection neuron traits and an as yet unknown signal for the induction of the photoreceptor fate. Via a small candidate screen, we recently identified BMP signalling as both necessary and sufficient to promote the photoreceptor fate. We have also demonstrate that crosstalk between BMP and Notch signalling is required for the inhibition of a projection neuron fate in future photoreceptors. In this case, BMP signalling is required as a competence factor for the efficient activation of Notch targets. Our results provide new insights into how multiple signalling influences are integrated during cell fate specification in the vertebrate CNS (Quillien et al., 2011).