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The interplay of active and passive mechanisms in slow axonal transport

A combination of intermittent active movement of transient aggregates and a paused state that intervenes between periods of active transport has been proposed to underly the slow, directed transport of soluble proteins in axons. A component of passive diffusion in the axoplasm may also contribute to slow axonal transport, although quantitative estimates of the relative contributions of diffusive and active movement in the slow transport of a soluble protein, and in particular how they might vary across developmental stages, are lacking. Here, we propose and study a model for slow axonal transport, addressing data from bleach-recovery measurements on a small, soluble, protein, Choline Acetyltransferase (ChAT), in thin axons of the lateral chordotonal (lch5) sensory neurons of Drosophila. ChAT is mainly present in soluble form in the axon and catalyses the acetylation of choline at the synapse. It does not form particulate structures in axons and moves at rates characteristic of slow component b (≈ 1-10 mm/day or 0.01-0.1 μm/s). Using our model, which incorporates active transport, paused and diffusive states, we predict bleach recovery and cargo trajectories obtained through kymographs, comparing these to experimental observations at different developmental stages. We show that changes in the diffusive fraction of cargo during these developmental stages dominate bleach recovery and that a combination of active motion with a paused state alone cannot reproduce the data. We compared predictions of the model with results from photoactivation experiments. The importance of the diffusive state in reproducing the bleach recovery signal in the slow axonal transport of small soluble proteins is our central result. STATEMENT OF SIGNIFICANCE While the fast axonal transport of cargo in axons is by now well-understood, the nature of slow transport remains controversial. A number of different models having been proposed for slow axonal transport, including models which allow for transitions between an intermittently moving molecular-motor driven state and a stalled state. How mechanisms for slow axonal transport are modulated during development is unexplored. We study a number of different models for slow axonal transport, comparing their predictions to data on transport of the enzyme Choline Acetyltransferase (ChAT) in thin lateral chordotonal (lch5) sensory neurons of Drosophila larva, across developmental stages where flux increases significantly. We show that accounting for changes in the diffusive fraction of cargo during these developmental stages is essential and diffusion cannot be neglected in the modelling of the slow axonal transport of small soluble proteins.