|dc.description.abstract||Multiple Sclerosis (MS) is a chronic inflammatory demyelinating disease of the central
nervous system (CNS), characterised by focal areas of myelin destruction
(demyelination), oligodendrocyte death with variable extent of axon damage
contributing to neurodegeneration. The aetiology of MS is not fully understood by the
scientific community, with considerations of a primary inflammatory root with an
autoimmune component as well of a degenerative disorder with secondary
inflammation under investigation.
Demyelination leads to conduction block of action potentials and to loss of metabolic
and structural support offered by the oligodendrocytes, contributing to axonal
vulnerability to damage and may in the long-term lead to neurodegeneration, the main
driver of permanent disability in MS patients. Regeneration of the myelin sheaths
(remyelination) around axons can help protect demyelinated axons from damage,
restore nerve conduction and thus function. Remyelination is carried out by
oligodendroglia, but in MS, this process is inefficient and promoting remyelination
represents a therapeutic goal due to the hopes to achieve prolonged neuroprotection.
In the healthy brain, interstitial tissue oxygen levels are low. Thus, CNS cells,
particularly oligodendrocytes and their precursors habitually exist in a physiological
low oxygen environment. To withstand shifts in the oxygen tension, such as mild
hypoxia, adaptive mechanisms take place in the brain, via the hypoxia inducible
factors (HIFs) and its target genes, that help sustain physiological functions with
changes in metabolism, angiogenesis, cell proliferation and cell death.
In humans and animal models of MS, neuropathological and imaging studies show
the presence of tissue hypoxia and evidence of tissue adaptations to this hypoxia.
Oligodendrocytes and their precursors are especially susceptible to hypoxia.
Remyelination, thus occurs within this hypoxic environment, but we do not understand
whether hypoxia worsens tissue repair or whether the tissue adaptations aid
remyelination, similarly to the regenerative effects of hypoxia pre-conditioning in other
organs and diseases.
To test this hypothesis, I used two experimental paradigms: 1) exposure to hypoxia
during remyelination and 2) exposure to hypoxia prior to demyelination, as pre-conditioning. To study remyelination, in the absence of an autoimmune response, I
used stereotactic injection of lysophosphatidylcholine (LPC) into the mouse corpus
callosum as a focal model of demyelination, exposed mice to mild hypoxia (10%
normobaric oxygen) and assessed remyelination efficiency at 10– and 15–days post
lesion (Dpl) by electron microscopy.
Firstly, I found that demyelinating lesions at 3 Dpl are hypoxic, as assessed using the
oxygen sensitive immunohistochemical probe, pimonidazole, and this hypoxia
persisted during remyelination at 10 and 15 Dpl.
I also found that exposure to additional hypoxia during the period of remyelination
resulted in a significant decrease in the number of remyelinated axons at 15 Dpl but
not at 10 Dpl, compared to mice undergoing remyelination in normoxia (room air).
Importantly, the observed decrease in the number of remyelinated axons was not due
to a loss of axons.
I also noticed a difference in thickness of the inner cytoplasmic tongue (ICT) than
expected for a given myelin thickness. The ICT is the inner most uncompacted myelin
layer facing the axon. The ICT is in direct connection to the oligodendrocyte soma
and via myelin ‘channels’ which provide passage of metabolites to the underlying
axons and this is the site of new myelin membrane deposition during myelin growth.
I observed that the ICT was larger than expected for myelin thickness at 10 Dpl in
remyelinated axons irrespective whether they were exposed to normoxia or hypoxia,
followed by its recovery at 15 Dpl in normoxic mice, compared to non-demyelinated
At 15 Dpl, mice that remyelinated under additional hypoxic conditions showed a larger
ICT for a given myelin thickness, compared to non-lesioned mice. This enlargement
of the ICT may represent a metabolic compensatory mechanism during early
remyelination in response to the lesion-associated hypoxia that may help sustain
remyelination through provision of metabolites. However prolonged hypoxia leads to
decompensation and together with an unfavourable lesion environment results in the
loss of myelin and poor remyelination.
I next assessed whether preconditioning of brain tissue to mild hypoxia before
inducing a focal demyelinating lesion affects subsequent remyelination using the
same model. Evidence from preconditioning paradigms in different organs, has shown
that this confers resilience to further damage, aiding tissue repair. Lesion size was
significantly reduced in mice preconditioned to hypoxia compared to controls, but
there was no difference in the number of remyelinated axons at the two post-lesion
In conclusion, this work provides evidence that additional longer period of hypoxia
during the period of remyelination may be detrimental to remyelination and that
hypoxia preconditioning warrants future investigation to fully address its possible
protective effect in reducing lesion size.||en