Exploring the relationship between gene expression and neuronal wiring in the developing Drosophila brain
De relatie onderzoeken tussen gen expressie en neuronale netwerken in de hersenen van Drosophila tijdens de ontwikkeling
Nicolaï, Laura; M0319797
One of the basic goals of research in neurobiology is to understand how the nervous system isbuilt. Neurons are compartmentalized into two morphologically and functional distinct compartments, axons and dendrites. Axogenesis and axon targeting have been extensively studied. But also dendritic morphogenesis is crucial to the proper formation of neuronal circuits. Gathering more information on the formation and maintenance of functional neuronal networks will also help disease-related research such as methods to repair or regenerate neurons after damage to the brain or spinal cord. The aim in the first part of my work was to develop a genetically encoded marker for the somatodendritic compartment in Drosophila. In studies of neuronal networks it is crucial to distinguish the axon from the dendrites and an efficient, neutral marker of the somatodendritic compartment was lacking in the fly. A marker, which we named DenMark, was developed, consisting of the mouse cell adhesion molecule ICAM5/Telencephalin and the red fluorescentprotein mCherry. The expression of ICAM5 is restricted to the somatodendritic compartment of the telencephalon in the mammalian brain and there is no homolog in invertebrates and lower vertebrates. The remarkable result of our research was that this cell adhesion molecule localizes with an exquisite precision to the somatodendritic compartment in Drosophila neurons. This raises the exciting possibility that DenMark may also serve as a dendritic marker in other model organisms such as Xenopus and/or zebrafish, in addition to Drosophila and mouse. Furthermore we showed that it does not cause any visible phenotypes in the neurons where it is expressed. Interestingly, using DenMark and other markers to compare dendritic development in Drosophila and mouse neurons reveals that dendrites do not acquire their complete molecular specialization until a relatively late stage in neuronal development. From our results and the current knowledge, we believe that dendrites in Drosophila are homologous to dendrites in vertebrates and by investigating the flys dendrites the findings can be extrapolated to vertebrates. This marker is a useful tool to investigate dendritic morphology, development, maturation and connectivity. Even though this dendrite-specific protein (ICAM5) is not preserved in evolution, the mechanisms used to sort proteins to the dendritic compartment appear conserved. In the second part, we investigated how gene networks control neuronal networks. First we focused on how one transcription factor, atonal, can orchestrate the correct formation of the developing optic lobe, while being only expressed in the R8 precursor and the T4/T5 neuroblasts. Since ato is highly conserved throughout evolution, these findings could provide more insight not only on the development of the visual system but also of other neural networks in different organisms. The role of ato in the developing retina is well documented, but in the optic lobe, more specifically in the T4/T5 neuroblasts, it seems to play several roles very different from its proneural function in the R8 precursors. We hypothesize that ato acts through different pathways to keep the cell bodies from migrating aberrantly inthe developing optic lobe, to correctly target the neurites of the offspring, and perhaps to help the cells to survive. The JNK pathway also plays a role in the optic lobe development and it is likely activated by DTRAF1. Loss-of-function of DTRAF1 in the T4/T5 neuroblasts of the developing optic lobe causes a similar phenotype in the adult brain, namely an overextension of the neurites in either medulla or lobula. Since ato acts upstream of the JNK pathway in the developing retina, we hypothesized it might also regulate this pathway in the inner optic lobe. Our attempt to rescue the ato null phenotype with DTRAF1 overexpression was not convincing, suggesting that DTRAF1 is not downstream of ato in the developing optic lobe as it is in the retina or that DTRAF1 alone is not sufficient to rescue the phenotype. This primary analysis of the development of the inner optic lobe proves that ato plays multiple roles to orchestrate the development of the visual system and furthermore that DTRAF1 is present in the developing IPC and plays a pivotal role via the JNK pathway in the developmentof the inner optic lobe.