Trafficking and Neurotrophic Effects of Botulinum Neurotoxin Type A

Abstract

Botulinum neurotoxin type A (BoNT/A) is a SNAP-25-cleaving protease produced by the bacterium C. botulinum that blocks synaptic transmission at the neuromuscular junction. Inhalation of 1 g can be lethal while lower doses can result in a paralysis that lasts for several months. However, this terrifying potency and persistence has converted it into a valuable drug in many muscle and neurological disorders. Paradoxically, BoNT/A induces nerve regeneration
and BoNT/E, a BoNT/A homolog, has a much shorter persistence despite many mechanistic similarities.
Tools for the comparative study of BoNT/A and BoNT/E were produced but shown to be ineffective for purposes of this thesis.
By using a catalytically inactive mutant of BoNT/A (BoNT/A(0)), I showed that the toxin enters neurons through multiple mechanisms before entering the endocytic pathway. BoNT/A(0) escaped lysosomal degradation but was degraded by the proteasome. Furthermore, a fraction of BoNT/A was static at the early endosome and another fraction was exocytosed and was able to re-enter neurons as a full-length toxin.
In addition, this research shows that neurite outgrowth induced by BoNT/A(0) is restricted to axonal outgrowth in vitro and is mediated by activation of the small GTPase Rac1 through the binding domain of BoNT/A (HCC/A). HCC/A also induced the formation of filopodia presynaptic vesicle release and neurogenesis.
This thesis provides a model for the trafficking of full-length BoNT/A prior to any catalytic action and reformulates BoNT/A receptor binding domain as a neurotrophic factor.

Date of Award	19 Mar 2019
Original language	English
Awarding Institution	University of Bristol
Sponsors	Ipsen Bioinnovation Ltd.
Supervisor	George Banting (Supervisor) & Jeremy M Henley (Supervisor)