Mitochondrial import failure and rescue

: links to neurodegeneration

Abstract

Protein import into mitochondria is an intricate and highly conserved process central to normal cellular function, especially given the fundamental interplay between mitochondrial protein import and respiratory complexes assembly and function. This is particularly important in cells with high energetic demands such as neurons. Consequently, it is not surprising that defects in mitochondrial import are often observed in models of neurodegeneration. However, relatively little is known about the precise mechanism(s) by which dysfunctional import contributes to neurodegeneration.
To facilitate better understanding of import processes in health and disease, I developed an assay system to monitor import in real-time in live mammalian cells, using NanoLuc technology. Intriguingly, whilst import efficiency was diminished following acute stalling of a precursor during translocation, there was no apparent change in import function in cells exposed to chronic precursor trapping, despite alterations in mitochondrial morphology and dynamics. Disease prone Tauᴾ³⁰¹ᴸ variant appeared to associate with TOM40 and induced mitochondrial changes resembling those observed with precursor trapping, indicative of a common mechanism. Both insults correlate with reduced neuronal complexity and synapse abundance, resembling phenotypic changes characteristic of neurodegeneration.
Additionally, this thesis describes how a cellular rescue mechanism compensates for import perturbations in cells exposed to a stalled precursor or Tauᴾ³⁰¹ᴸ-TOM40 association. Perturbing import induced the formation of tunnelling nanotubes (TNTs) which rescue import function by enabling intercellular mitochondrial transfer. TNT formation was also induced by a small molecule TIM17 inhibitor, MB20, demonstrating that this could represent a widespread response to import dysfunction.
Overall, the work presented in this thesis demonstrates how a novel assay system can be exploited to advance knowledge of the mechanisms linking mitochondrial import defects and neurodegeneration. Furthermore, it shows how a known stress response mechanism, mitochondrial transfer via TNTs, is activated in response to failed import, and proposes an intriguing link between Tau and mitochondrial import relevant to disease.

Date of Award	21 Jun 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Wellcome Trust
Supervisor	Ian R Collinson (Supervisor) & Jeremy M Henley (Supervisor)