Mitochondrial trafficking dynamics in axons

: insights from in vivo two-photon imaging and computational modelling

Abstract

Mitochondria are intracellular organelles, which support neurons by providing energy and buffering calcium. In extended cellular architectures such as axonal arbors, trafficking is crucial to distribute and dynamically position mitochondria at sites of demand. Mitochondria are known to support neurotransmission, and, in turn, neuronal activity can regulate their trafficking. In vitro studies have shown that short-term changes in neuronal activity dynamically alter mitochondrial motility. With advances in in vivo imaging, mitochondrial dynamics have been studied in functional contexts revealing differences in motility characteristics, thus raising questions on the nature of mitochondrial transport regulation in the living brain. In this thesis, computational modelling and in vivo two-photon microscopy were used to investigate the dynamics of mitochondrial trafficking in long-range axons.

A stochastic single-particle model of axonal mitochondrial trafficking was developed and fitted to in vivo two-photon imaging data of mitochondria in distal cortical axons. Simulations showed that the latency to distribute mitochondria was sensitive to bidirectional transport bias, offering a trade-off between delivery speed and cargo accumulation at distal ends. Additionally, longer axons faced extended delays in delivery from the soma to their distal ends, which was further aggravated by impairments in trafficking related to tauopathy. Results suggest long-range axons may be more vulnerable to defects in mitochondrial transport.

In vivo two-photon microscopy experiments explored the activity-dependent regulation of mitochondrial trafficking. Mitochondria were tracked in distal axons before and after pharmacologically silencing action potentials. Although neuronal silencing significantly increased the proportion of mobile mitochondria without obviously influencing their transport patterns, over 90% remained stationary. This suggests that neuronal activity may not be the primary regulator of mitochondrial anchoring along the axon, despite its influence on motility. These results highlight the necessity to further investigate neuronal control of mitochondrial trafficking in structurally and functionally relevant systems.

Date of Award	10 Dec 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael Ashby (Supervisor), Cian O'Donnell (Supervisor) & Jack R Mellor (Supervisor)