Vascular cognitive impairment describes a heterogeneous condition in which cognitive decline
is precipitated by underlying cerebrovascular dysfunction. Ageing, as well as vascular diseases
such as hypertension, stroke, cerebral small vessel disease and cerebral amyloid angiopathy,
are risk factors for vascular cognitive impairment. The precise mechanisms by which these
conditions impact the cerebral vasculature to drive cognitive decline, however, are unknown.
Previous research has indicated that vascular risk factors can lead to microvascular oxidative
stress, inflammation and endothelial dysfunction that can lead to tissue hypoperfusion, the
development of white and grey matter vascular lesions (microinfarcts and microbleeds) and
cognitive impairment. It was hypothesised that ageing, a prominent risk factor for cognitive
decline, would induce impairments on neurovascular coupling resulting from neurovascular
unit disruption. It was further hypothesised that induction of chronic cerebral hypoperfusion
would mediate neurovascular dysfunction and vascular lesion development through increased
oxidative stress, resulting in cognitive decline. Finally, it was also hypothesised that
neurovascular impairments resulting from ageing and chronic cerebral hypoperfusion would
be exacerbated in the presence of amyloid deposition. Four studies were performed in order to
test these hypotheses.
Vascular risk factors can be reproduced using experimental mouse models and provide a
valuable basis in which to test hypotheses and therapeutic interventions. As such, a primary
aim of this thesis was to develop and validate sensitive MRI approaches that would allow the
detection of vascular alterations in vivo. In the first series of studies, MRI techniques to assess
resting cerebral blood flow, vessel number, vascular lesions and inflammation in experimental
mice were validated using established in vivo and ex vivo techniques, so that these techniques could be used in subsequent studies for vascular assessments in vivo. Arterial spin labelling
was developed to assess resting cerebral blood flow, and was able to detect reductions in blood
flow following cerebral hypoperfusion that correlated well with those obtained from laser
speckle imaging. Q-map imaging was able to detect reductions in vessel number in acute
lesions, and in non-lesioned mice measures of vessel number correlated well with
histopathological measures. Structural T2 imaging was performed in order to detect ischaemic
and haemorrhagic lesions in chronically hypoperfused mice, and was validated using H&E and
Perls’ staining. Finally, contrast-enhanced T2* imaging was used to detect iron oxide uptake
by macrophages in the brains of hypoperfused mice, which was further validated by the
identification of iron-containing macrophages in immunostained brain sections.
The second study was conducted to test the hypothesis that ageing would impair neurovascular
unit function and structure, and that these impairments would be exacerbated in the presence
of amyloid pathology. The aim of the study was to incorporate previously developed in vivo
imaging approaches in the assessment of vascular function and alterations in neurovascular
unit structure in both wild type and TgSwDI mice. As predicted, ageing caused a pronounced
deficit on measures of neurovascular coupling, however this was not exacerbated by
accumulation of amyloid in TgSwDI mice and was not associated with alterations in baseline
blood flow measured by arterial spin labelling. Structural assessment of the neurovascular unit
revealed a loss of contact between astrocytic endfeet and vasculature, which was significantly
associated with the impairment on neurovascular coupling, in addition to other markers of
breakdown of the neurovascular unit such as loss of pericyte coverage and microglial
activation. Age and thalamic vascular amyloid accumulation were also associated with an
increase in the NADPH oxidase (NOX) subunit p47, indicative of increased oxidative stress. Data from this experiment indicate that ageing can profoundly impair neurovascular coupling,
mediated by gliosis and loss of astrocytic contacts with vasculature.
The third study aimed to test the hypothesis that chronic cerebral hypoperfusion (a prominent
early feature of vascular cognitive impairment) would impair vascular function and induce the
development of vascular lesions and cognitive decline. The impact of hypoperfusion on
neurovascular coupling, ischaemic and haemorrhagic lesion burden and cognition was
investigated in wild type and TgSwDI mice. Hypoperfusion induced deficits on neurovascular
coupling, increased lesion burden and inflammation assessed with T2 and contrast-enhanced
T2* imaging, and caused impairment on measures of learning and memory. Hypoperfusion
was also associated with an increase in the levels of NOX2, NOX4 and 3-NT at 3 months
following surgery, indicating persistent reactive oxygen species production and oxidative
damage in hypoperfused mice. The findings from this study indicate that vascular dysfunction
and cognitive impairment following hypoperfusion may be mediated by increased NADPH
oxidase activity and resulting oxidative stress.
The previous studies indicated that markers of oxidative stress were induced in response to
ageing, vascular amyloid accumulation and cerebral hypoperfusion. The final study sought to
determine whether increased NOX activity mediates downstream pathological effects on
vascular function, vascular lesion development and cognitive decline following hypoperfusion.
NOX activity was inhibited pharmacologically by administration of apocynin to hypoperfused
TgSwDI mice for 3 months following surgery. Treatment with apocynin significantly restored
neurovascular coupling to a level similar to sham-operated mice, and there was a trend toward
reduction of ischaemic vascular lesions. However, it was unable to rescue the prominent inflammatory response or decline in cognitive ability, as apocynin-treated mice were no
different on these measures to non-treated hypoperfused mice. The data indicate that whilst
inhibiting NOX may have potential therapeutic value in improving vascular function,
additional interventions, for example to reduce inflammation, may also be required in order to
prevent cognitive decline.
Overall, the work outlined within the thesis indicate that vascular risk factors of ageing,
cerebral amyloid angiopathy and cerebral hypoperfusion may converge on common pathways
involving oxidative stress and increased inflammation in order to drive vascular dysfunction
and lead to cognitive decline. Inhibition of NOX activity was able to rescue vascular function,
however the results indicate that this was not sufficient to protect against cognitive impairment,
suggesting additional therapeutic targets may need to be sought in order to fully preserve
vascular health and prevent cognitive decline