What regulates axonal diameters in the central nervous system?
Final Report Abstract
The vertebrate central nervous system (CNS) requires rapid action potential (AP) propagation along axons to function. Axons vary in diameter by more than 100-fold, from 0.1µm to over 20µm, influencing AP conduction: 1) Larger axons increase AP propagation speed. 2) Axons over 0.5μm in the CNS stimulate myelination, further speeding up conduction. This diversity in conduction speed is crucial for coordinating AP timing within and across neural circuits. Axon diameter adapts to changes in neuronal activity, indicating an underestimated form of neural circuit plasticity. Further, different axon sizes have varying susceptibilities to diseases and diameter distributions change in neurodevelopmental and neurodegenerative diseases. Despite its importance, the processes governing axon diameter variations are poorly understood. This project aimed to investigate the cellular and molecular bases of axon diameter growth in the CNS. Traditionally, axonal diameters have been studied ex vivo using light or electron microscopy. To investigate dynamic growth in vivo, I chose zebrafish due to their optical transparency, small size, rapid development, and well-defined circuitry with differently sized axons, visualisable using fluorescent reporters. After testing various reporters and imaging different axons, I focused on the Mauthner neuron and its easy tractable axon. Initial experiments suggest that target motor and sensory input neurons do not affect the Mauthner axon diameter. Thus, I focused on the molecular mechanisms regulating axon diameter growth. Leveraging zebrafish's discovery potential, I conducted a chemical phenotypic screen using the Sigma LOPAC library, containing 1280 drugs with well-described molecular targets. I used a zebrafish line with fluorescent expression in the Mauthner axon and a high-resolution automated imaging system to monitor axon diameter changes in vivo. We developed an automated data and image processing pipeline with machine learning algorithms. I validated this workflow using a chemical inhibitor of the mTor cell growth pathway, which inhibits axon diameter growth. This screen revealed molecular candidates impacting axon diameter growth, especially compounds targeting neurotransmission and ion channel modulation, supporting the hypothesis that neuronal activity influences axonal diameters. The results form the basis for ongoing investigations into molecular regulators of axon diameter growth, supported by a Marie Curie postdoctoral fellowship.
Publications
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Poster presentation at the XVI European Meeting on Glial Cells in Health and Disease, July 2023, Berlin, Germany. “A high-resolution in vivo drug-screen in zebrafish to investigate how myelinated axons grow in diameter.”
Maria Eichel-Vogel
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Myelinated peripheral axons are more vulnerable to mechanical trauma in a model of enlarged axonal diameters. Glia, 72(9), 1572-1589.
Gargareta, Vasiliki‐Ilya; Berghoff, Stefan A.; Krauter, Doris; Hümmert, Sophie; Marshall‐Phelps, Katy L. H.; Möbius, Wiebke; Nave, Klaus‐Armin; Fledrich, Robert; Werner, Hauke B. & Eichel‐Vogel, Maria A.
