Our latest work (previously on bioRxiv) is now published in the Journal of Cell Biology. We collaborated with the Roy lab to reveal a new mechanism of slow axonal transport, based on the previous discovery of actin hotspots and trails. Hotspots are static actin clusters that appear and disappear within minutes every 3-4 µm along the axon. They generate the assembly of trails, long actin filaments that polymerize along the axon and collapse within seconds. Our new article first shows that trails polymerize at their barbed ends, located at the surface of hotspots. Each trail is thus pushed away from the hotspot as as it grows, resulting in a net displacement of actin monomers after trail collapse. In addition, trails grow in both directions (anterograde and retrograde), but with a small bias toward the tip of the axon (58% anterograde vs 42% retrograde).
The combination of these two processes (displacement of actin by trails and anterograde bias) results in the slow progress of actin along the axon. Modeling from the Jung lab allowed to determine the overall actin transport speed resulting from the hotspots and trails dynamics. Strikingly, this slow anterograde transport speed of actin (0.4 mm/day) precisely matches the values obtained by classic radio-labeling studies. This is a fundamentally new mechanism of slow axonal transport for cytoskeletal components, based on a biased assembly/disassembly mechanism rather than processive transport by motor proteins.
In this work, we used STORM imaging of axonal actin to pinpoint the architecture of hotspots, showing that the multi-directional growth of trails make them appear as asters when the axon is thicker (see Figure). Furthermore, we imaged hundreds of hotspots by STORM and quantified their diameter to ~200 nm. This is a first step toward elucidating the molecular organization of hotspots and trails, which will be crucial to understand their cellular functions.
Christophe was interviewed by journalist Anne Debroise of the French scientific magazine Science & Vie about a recent article from the Bock lab (Zheng et al., 2018) that provides the volume of an adult Drosophila fly brain by electron microscopy. You can read the short article here (in French, subscription required) or via the screenshot below.
We have a new preprint out! It’s a collaboration with the group of Sandrine Lévêque-Fort at ISMO (Orsay, France) based on the PhD work of Clément Cabriel. They previously used supercritical-angle fluorescence to measure the height of fluorophores at the proximity of the coverslip. This technique, called Direct Optical Nanoscopy with Axially Localized Detection (DONALD), could bring the resolution down to 15 nm for 3D localization microscopy. Now they have combined the SAF-based method with cylindrical lens astigmatism to obtain a robust and precise 3D localization of fluorophores over ~1.5 µm above the coverslip, retaining the key advantafges of DONALD: drift-free, tilt-insensitive and achromatic. The new technique, called Dual-view Astigmatic Imaging with SAF Yield (DAISY 😉), allowed to image in 3D the periodic scaffold of adducin and ß-spectrin along axons of cultured neurons, as you can see on the Figure below:
We all said goodbye to Nikki who finished her Master’s internship at the end of July. Nikki is a great student, and she managed to get a PhD position in the lab of Ruud Toonen at the Center for Neurogenomics and Cognitive Research in Amsterdam. Good luck for this new adventure!
Christophe was in Nantes to talk at the first biennale of the Nikon Center of Excellence Nantes (CENN), devoted to “Super-resolution microscopy for Biology”. Diverse and interesting talks from Stefan Balint, James Zheng, Anne Beghin et al. in the beautiful Musée des Beaux-Arts of Nantes.
Christophe was lucky to share the stage with Ryohei Yasuda, Marina Mikhaylova and Sonja Hofer for a symposium dedicated to neuronal imaging. Thanks to Deepak Srivastava for the invite (the fish n’ chips was delicious)!
We had the chance of having Dominic Bingham, currently a Master’s student in Alison Twelvetree’s lab in Sheffield, apply for a PhD in the lab through the “Integrative and Clinical Neuroscience” PhD program. Congratulation to Dominic who succeeded to be one of the three laureates this year! We can’t wait to have him the lab where he will study the organization of presynaptic actin. Thanks also to the Aix-Marseille University A*MIDEX which is funding the PhD program.
It was a pleasure to present at the 7th Cell Adhesion Club Meeting in Strasbourg. After three fascinating days hearing about cell adhesion and related cell biological processes by top scientists of the field, it was very interesting to see how key tools and concepts can also inform cellular neurobiology. See the #Stradh18 Twitter hashtag for more, thanks to @GoetzJacky for the invite!
Marie-Jeanne and Christophe wrote a review detailing how recent discoveries renewed the understanding of axonal actin organization. In the axon shaft itself, new nano-structures such as rings, hotspots and trails have been described, but their function remains to be elucidated. At presynapses, the precise architecture of actin is still elusive, and contradicting findings have been reported regarding its function. This is an exciting time to study actin in axons!
The review is now published in Molecular and Cellular Neuroscience, and will be part of a special issue on “Membrane Trafficking and Cytoskeletal Dynamics in Neuronal Function”. If you don’t have access to the review, a preprint manuscript is available on Zenodo.