Anti-GD1a antibody targeted disruption of the node of Ranvier in a mouse model of acute motor axonal neuropathy.
PhD thesis, University of Glasgow.
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Guillain-Barré syndrome (GBS) is a peripheral neuropathy characterised by
acute flaccid paralysis. The axonal variant is associated with anti-GD1a
ganglioside antibody-dependent, complement-mediated injury to the peripheral
axon with conduction block. The blood-nerve barrier (BNB) relatively protects
axons from factors in the extra-neural environment; however, it does not extend
over the neuromuscular junction, leaving this terminal portion of the axon
unprotected. It is here that susceptibility to antibody attack in a mouse model of
GBS has previously been demonstrated. It was the aim of this thesis to
determine to what extent more proximal portions of the distal axon are at risk
from circulating antibody and what exogenous protection can be provided
GD1a is expressed at the nodes of Ranvier (NoR) of intramuscular axons.
To investigate the injury caused by anti-GD1a antibodies in relation to BNB
permeability, anti-GD1a antibodies were applied to mice genetically engineered
both to over-express GD1a and to express cyan fluorescent protein (CFP) in the
cytoplasm of axons. Endogenous fluorescence allowed identification of
intramuscular nerve bundles and their terminal branches, which were
categorized depending on bundle size. Within these categories, IgG and the final
product of the complement pathway (membrane attack complex, MAC)
deposition were quantified after an acute injury, alongside the deleterious
effects on NoR protein’s. Nerve conduction studies were also performed to
better elucidate the pathological pathway.
IgG and MAC were localized in a gradient-dependent manner, with
significantly more deposition at NoR as the bundles progressively branch to a
single terminating fibre. Furthermore, MAC deposition was associated with the
loss or disruption to immunostaining for nodal protein’s including voltage gated
sodium channel and ankyrin G. This is indicative of targeted injury to this region
of the distal axon in an acute model. The loss of nodal protein staining is
associated with the activation of complement and the Ca2+-dependent protease
calpain as determined by the protection of staining by the complement inhibitor
Eculizumab and the calpain inhibitor AK295. A similar disruption to nodal protein
staining is also shown at the proximal NoR of the desheathed phrenic nerve.
Extracellular nerve recordings demonstrate a detrimental effect on
function as there is a decrease in the peak of the compound nerve action
potential over time, which can be associated with Nav channel staining loss.
This study is suggestive of a resilient proximal barrier that becomes more
permeable towards the nerve terminal. Therefore, it is not only the axon at the
terminal that can be a target of injury, but also the distal axons at their nodes
of Ranvier, resulting in disruption at this site. Prevention of staining loss by
Eculizumab and AK295 exemplify the route of injury and identify a potential
point of therapeutic intervention in human disease.
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