The mechanistic basis and potential disease relevance of microtubule disorganisation in axons

点击翻译按钮获取中文摘要

Here we will study the properties of the microtubule (MT) cytoskeleton of neuronal axons to gain a better understanding of the important roles that MTs play during the formation, maintenance and degeneration of neurons. Axons are the slender, cable-like, up to several meter long protrusions of neurons which form the nerves that electrically wire our bodies. They can usually not be replaced, hence need to be maintained for up to a century in humans. Unsurprisingly, we gradually lose ~50% of our axons towards old age - and far more in neurodegenerative diseases (ND). In spite of their enormous importance, we know far too little about the mechanisms that maintain these delicate structures long-term or lead to their premature decay in neurodegeneration. Axon formation and maintenance essentially depends on the microtubule (MT) cytoskeleton. MTs consist of filamentous protein polymers arranged into 25nm thick tubules. In axons, MTs form continuous parallel bundles, serving as structural backbones and highways for life sustaining cargo/organelle transport. In ageing and ND, axons often form swellings where MT bundles become disorganised into criss-crossing curls, which trap organelles and are believed to trigger axonal loss. For the study of MT disorganisation, we developed the working model of axonal homeostasis based on our experimental data obtained in Drosophila neurons; this model involves three steps (details in CfS pt. 1): (1) MTs have to undergo constant polymerisation/depolymerisation to self-renew. (2) Each MT polymerisation event poses a risk of MT disorganisation, particularly in axons where high densities of MTs and molecular motors generate shear forces which can induce MT curling. (3) Order is imposed by a range of different mechanisms mediated by MT-binding proteins (e.g. by guiding MTs into parallel bundles, or eliminating MTs that have gone off-track). We propose that loss of single (or multiple) of these order-imposing mechanisms increases the risk of MT disorganisation leading to axon swellings - thereby providing potential explanations for late-onset axon degeneration linked genetically to various MT regulators. So far we have tested and refined this model primarily through experimental work in cultured fly and mouse neurons, by focusing on mechanisms regarding step 3 of our model (i.e. order-imposing MT regulators). Here we will focus on the mechanisms involved in step 2 (i.e. causing the curling of MTs), and compare our knowledge in cultured neurons to the situation in the nervous system in vivo. This work is important for several reasons: First, data obtained here will reveal the degree to which observations made in the highly efficient model of cultured neurons, reflect mechanisms underlying axon swellings in vivo. This will give important direction for experimental work aiming to unravel how axon swellings form and can be prevented. Second, we will generate important data concerning MT dynamics and their spatial arrangements in axons. These data will provide important information for the mathematical models of MT behaviours (see support letters) which we are developing in parallel projects - aiming to eventually perform long-term in silico experiments that can test pathological roles of MTs in late-onset neurodegeneration.Third, our data will provide important understanding, descriptions and concepts of axonal MTs that will aid worldwide research into axonal transport, organelle dynamics and MT regulation, thus promoting general advances in our understanding of axon biology during development, ageing, regeneration and degeneration.