Section: New Results

Axon Growth Imaging and Modeling

Participants : Agustina Razetti, Caroline Medioni, Florence Besse, Xavier Descombes.

The modeling part of this work has been made in collaboration with S. Komech, E. Pechersky and E. Zhizhina from IITP (Russian Academy of Science)

In Drosophila brain, at metamorphosis, Mushroom Body gamma neurons undergo axonal remodeling characterized by a pruning of larval branches followed by regrowth and branching/arborization of adult processes. Axonal regrowth at this stage is essential to consolidate the adult brain and its success is determined by the trajectories followed by the axons and their branches. These trajectories depend on both extracellular guidance signals, and on a complex internal molecular machinery capable to read these signals and act in consequence. F. Besse's team at the IBV Institute has identified genes involved in this regrowth and branching processes [19] . A better understanding of the role of these genes will help to unravel the molecular mechanisms behind these fundamental processes, and lead to a better understanding of the neuronal morphology in both healthy and pathological conditions.

During this PhD project, mathematical and computational tools will be developed to characterize and compare the axonal regrowth and branching dynamics. Different populations of regrowing gamma axons will be considered (i.e. wild type and presenting mutations in relevant genes). The study will be based on both static 3D confocal images of axonal trees, and two-photon in vivo 4D image sequences showing either a single GFP positive regrowing axon or the entire population of regrowing gamma axons marked with GFP. In a first part we have begun to collect data. This includes three parts: i) fly stock maintenance, crossing and selection; ii) sample preparation consisting in pupal brain dissection, medium preparation and sample final assembly; iii) imaging: using different microscopy techniques, eg.confocal/two-photon, microscopy and light sheet microscopy, and acquisition of 4D image sequences. Thanks to an imaging technique developed by C. Medioni in F. Besse’s laboratory, we have been able to monitor axonal regrowth and branching at early steps in individual neurons, and to follow them for about 15 hours using the two-photon microscope. These movies will be used in the future to develop the mathematical modeling of axonal regrowth/branching process (see figure 3 ). Our early works concerning modeling have consisted in investigating some models based on continuous time random walks and characterizing the main axon branch through topological entropy [8] [22] .