Control of ventral neuronal cell differentiation by a novel protein

Brain tumours are dramatic not only because of their medical impact on individual health but also because of their social and economical consequences on individuals and on the public. This project is interested in the role of a growth factor, Sonic hedgehog (Shh), whose implication in brain tumour formation and growth is well-documented. This growth factor also plays important functions during the normal development of the Central Nervous System and it is believed that a deregulation of its normal function is at the origin of tumour formation. Thus, to better understand how tumours form, it is essential to gain knowledge in the mechanisms of action of this growth factor in a normal context. This is what this research aims at. We have found a novel protein that appears to carry out some of the functions of the growth factor Shh and we hypothesized that it is controlling the decision to stop growing and mature into neurons. Our objectives are to gain more insights into the role of this protein and establish the exact step(s) this protein is involved in during Central Nervous System development. Data flowing from this study will provide an important missing link in the steps that control the normal development of the Central Nervous System. This will be invaluable to understand brain tumour formation and design potential therapeutic strategies.

Upon completion, this study will be made publicly available to the specialist readership (researchers and doctors) through presentations at national and international conferences and publication in peer-reviewed journals, and to the more general readership through review articles and talks to students.

There has been a wealth of studies in Sonic hedgehog (Shh) signalling and its function in the Central Nervous System. However, despite the efforts invested in this area of research, little is known of the mechanisms that underlie the diversity of responses triggered by Shh signals. For instance, Shh signalling is required to establish distinct neuronal progenitor domains in the ventral neural tube and is also necessary for the proliferation of neural progenitor cells. We believe such pleiotropic effects are established through the activation of downstream target genes, whose function is to interpret Shh signals for a specific response.
To gain further insights into this process, we have screened subtracted cDNA libraries to identify Gli target genes and we isolated a putative Shh response gene. Our preliminary data show that it is specifically expressed in the developing Central Nervous System (CNS) and gut, and encodes a novel armadillo repeat-containing protein. We have confirmed its regulation by Shh signalling and using gain-of-function studies in the chick embryo, we found that it plays a critical role in ventral neuronal progenitor cells of the neural tube in the control of cell cycle exit and decision to differentiate.
This proposal aims at investigating further the role of this novel protein downstream of Sonic hedgehog signalling in the control of the proliferation/differentiation decision point made by neuronal progenitor cell of the ventral neural tube. The main objectives are 1) to establish whether this novel gene functions in one or more ventral progenitor cell types, 2) to determine its exact function, 3) to uncover the protein domains that are essential to carry out its function in the CNS.
To achieve these aims, shall investigate by immuno-histochemistry the effect of gain and loss-of-function studies on the proliferation of ventral neural progenitor cells, on the organization of ventral progenitor domains, and on the subsequent differentiation of the various ventral neurons that have been shown to be dependent on Shh signalling. Deletion and mutation constructs that affect the armadillo-repeat domain of the protein will be generated and their function tested in both cell culture and in vivo approaches. Together, these data will provide a missing link between Shh signal transduction and specific outputs in the Central Nervous System. Such links are essential if one wishes to understand the mechanisms underlying CNS tumour formation and uncover potential targets for drug and therapies.