Axon morphogenesis and local RNA regulation in Drosophila

By extending to specific locations and branching in a specific manner, neuronal processes determine how the nervous system is wired to produce behavior. Axonal projections, in particular, acquire a complex and cell type-specific morphology controlled by a precise developmental program, and dictated by both intrinsic and extrinsic cues. Recent work has suggested that local post-transcriptional regulation of mRNAs encoding proteins such as receptors to guidance cues or cytoskeleton regulators is essential for axon growth and guidance in vitro.

We are using Drosophila adult Mushroom Body γ neurons as a model system to study the importance and the mechanisms underlying post-transcriptional regulation during axon morphogenesis in vivo. We have recently performed a selective loss-of-function screen to identify novel factors controlling this process, and have identified two conserved RNA binding proteins regulating axon maintenance (Marchetti et al., 2014), or axon growth and guidance (Bruckert, Marchetti et al., 2015).

 
Figure 1 - Wild-type and mutant axonal projection patterns

Figure 2 - Transport of neuronal granules in intact brains

We have also recently shown that Drosophila Imp, the sole fly member of a family of conserved proteins involved in subcellular targeting of mRNA, is required for the remodeling of Mushroom Body γ axonal branches that occurs during metamorphosis (Medioni et al., 2014) (Figure 1). Using a novel live-imaging protocol (Medioni et al., 2015), we have further demonstrated that Imp is transported to MB γ axons as particles undergoing microtubule-dependent motion during metamorphosis (Figure 2). Combining biochemical purifications and genetics, we identified profilin mRNA as a direct and functional target of Imp that colocalizes with Imp in axonal ribonucleoprotein granules. This work uncovered a role of mRNA transport in developmentally programmed axonal growth and branching, and revealed that mRNA targeting machineries undergo precise spatio-temporal regulation during brain maturation.


Figure 3 - Cultured neuron with neuronal
granules labeled in green

Although neuronal ribonucleoprotein granules appear to regulate fundamental neuronal processes, how these granules are assembled and regulated is still unclear. Using Imp neuronal granules as a paradigm, we now aim to understand how the transport, the assembly and the turnover of Imp neuronal granules is regulated. We address these questions using a variety of approaches, ranging from fly genetics and biochemistry to high-throughput imaging or modeling.

 

Ongoing projects include:

  • biochemical and genetic dissection of the determinants underlying Imp granule homeostasis and transport to axons
  • genome-wide RNAi screen to identify genes regulating Imp granule assembly (in collaboration with the DRSC; Figure 3)
  • biochemical purification of Imp complexes to identify members and regulators of Imp granules
  • computational modeling to better understand the function of imp in axon growth and branching (in the context of the Morpheme team).
  • functional and cellular analysis of imp-dependent axonal remodeling

 


Figure 4 - High-throughput microscopy screen

Understanding how neuronal granules are regulated is also critical to understand the etiology of neurodegenerative diseases, as recent lines of evidence have suggested that the progression of spinocerebellar ataxias, amyotrophic lateral sclerosis, or spinal muscular atrophy is linked to the accumulation of pathological RNP aggregates. In this context, we participate to a European project funded by the JPND to purify and compare the composition of RNP granules assembled by RNA binding proteins involved in ALS and SMA.

Last publications

The Secret Life of RNA: Lessons from Emerging Methodologies. - 2018 - Methods in molecular biology (Clifton, N.J.) - 1649 P1-28 - Medioni C, and Besse,F

Sumoylation regulates FMRP-mediated dendritic spine elimination and maturation. - 2018 - Nature communications - 9 P757 - Khayachi A, Gwizdek C, Poupon G, Alcor D, Chafai M, Cass F, Maurin T, Prieto M, Folci A, De Graeve F, Castagnola S, Gautier R, Schorova L, Loriol C, Pronot M, Besse F, Brau F, Deval E, Bardoni B, and Martin,S

TrawlerWeb: an online de novo motif discovery tool for next-generation sequencing datasets. - 2018 - BMC genomics - 19 P238 - Dang LT, Tondl M, Chiu MHH, Revote J, Paten B, Tano V, Tokolyi A, Besse F, Quaife-Ryan G, Cumming H, Drvodelic MJ, Eichenlaub MP, Hallab JC, Stolper JS, Rossello FJ, Bogoyevitch MA, Jans DA, Nim HT, Porrello ER, Hudson JE, and Ramialison,M

Neuronal RNP granules: from physiological to pathological assemblies. - 2018 - Biological chemistry - 399 P623-635 - De Graeve F, and Besse,F

Linking amyotrophic lateral sclerosis and spinal muscular atrophy through RNA-transcriptome homeostasis: a genomics perspective. - 2017 - Journal of neurochemistry - 141 P12-30 - Gama-Carvalho M, L Garcia-Vaquero M, R Pinto F, Besse F, Weis J, Voigt A, Schulz JB, and De Las Rivas,J

Live imaging of axonal transport in Drosophila pupal brain explants. - 2015 - Nature protocols - 10 P574-584 - Medioni C, Ephrussi A, and Besse,F

Drosophila Hrp48 Is Required for Mushroom Body Axon Growth, Branching and Guidance. - 2015 - PloS one - 10 Pe0136610 - Bruckert H, Marchetti G, Ramialison M, and Besse,F

The TRIM-NHL Protein Brat Promotes Axon Maintenance by Repressing src64B Expression. - 2014 - The Journal of neuroscience : the official journal of the Society for Neuroscience - 34 P13855-64 - Marchetti G, Reichardt I, Knoblich JA, and Besse,F

Imp Promotes Axonal Remodeling by Regulating profilin mRNA during Brain Development. - 2014 - Current biology : CB - 24 P793-800 - Medioni C, Ramialison M, Ephrussi A, and Besse,F

Principles and roles of mRNA localization in animal development. - 2012 - Development - 139 P3263-76 - Medioni C, Mowry K, and Besse,F

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Besse Florence
Group Leader

2009 HFSPO Award

2008 ATIP CNRS

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Members Team
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   De Graeve Fabienne
   Medioni Caroline
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   Formicola Nadia
   Pushpalatha Kavya Vinayan
   Razetti Augustina
   Vijayakumar Jeshlee
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    October 2018 - Florence Besse Team
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    June 2014 - F. Besse Team
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    July 2012 - Florence Besse Team
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    October 2009 - F. Besse Team
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    January 2009 - F. Besse team Team
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