Deciphering the function of RFX in morphogenesis of the neuronal network that controls growth and feeding behavior in Drosophila – Obeli-X

Drosophila provides a simple and powerful genetic model to study how neuronal networks are established and to dissect their functions. In particular, Drosophila genetic studies brought important contributions in understanding how some specific neuronal networks coordinate nutrient status to feeding behaviors, energy consumption and growth control.
We have identified a novel subset of Drosophila brain neurons that plays a major role in controlling feeding behavior. These neurons are characterized by the expression of a brain specific isoform of the RFX protein, a member of the RFX family of transcription factors. These factors have been shown to control the expression of genes involved in cilia assembly in C. elegans, Drosophila and mammals. Interestingly, we observe that flies deficient for the RFX brain isoform show over-growth, developmental delay and altered transition in feeding behaviors during the third larval instar. These phenotypes are associated with altered dendritic arborization and axonal growth of RFX expressing neurons (RFX+ neurons). Our working hypothesis is that RFX is required for neuronal morphogenesis of a subset of neurons in the Drosophila brain that are required to coordinate feeding behaviors, development and metabolic responses.
The present project aims to understand how RFX controls neurite morphogenesis in the Drosophila brain and how RFX+ neurons control feeding behavior in Drosophila. Our first goal will be to identify and characterize the genes that are regulated by RFX in the Drosophila brain. This will allow us to understand how RFX transcription factors control neurite morphogenesis and to identify novel genes involved in neuronal differentiation. It will also give important information on the molecular signatures of RFX expressing neurons. The second goal is to understand how RFX specificity is governed and to what extent RFX target genes in the brain overlap with RFX target genes involved in ciliogenesis in the peripheral nervous system (PNS). The brain and PNS RFX isoforms share the identical characteristic protein domains of this family and in particular an identical DNA binding domain, suggesting that target specificity is accomplished by cofactors. To achieve this goal, we will search for RFX partners by genetic approaches. Last, because these RFX expressing neurons define a novel subset of neurons in the Drosophila brain, our third goal is to understand how these neurons are connected to the ones that have already been described to be involved in growth and feeding behaviors in Drosophila.
Several observations suggest a role of ciliary protein in neuronal cell physiology. Indeed, human diseases resulting from the dysfunction of proteins involved in ciliary assembly are associated with impaired neuronal functions and several proteins required for cilia assembly are also required for neuronal physiology. Through the characterization of RFX function in the Drosophila brain this project will also determine whether proteins known to have a role in cilia assembly and to be regulated by RFX in the peripheral nervous system, may also play a role in neurons of the Drosophila brain.
In summary, our project is based on novel insights supported by our recent investigations and relies on powerful genetic approaches in Drosophila. It will address the fundamental question of the function of the RFX transcription factors in neuronal cell physiology. It will also help to elucidate the regulatory loop that goes from nutrient sensing to physiological and metabolic responses in Drosophila. It will have scientific spin-offs to understand human diseases associated with defective axonal growth. Last, it will address the question of the link between ciliary proteins and neuronal dysfunction in several human syndromes associated with defects of cilia associated proteins.