Chronic cerebral hypoperfusion: a key mechanism leading to vascular cognitive impairment and dementia. Closing the translational gap between rodent models and human vascular cognitive impairment and dementia.
Summary (5 min read)
Introduction
- Vascular disease has been invariably linked to cognitive impairment.
- Several previous reports, including recent ones from the AD research centres in the USA, suggest that some form of brain vascular pathology exists in up to 80% of sporadic late onset AD (6) .
- Traditional risk factors for stroke and cardiovascular disease (e.g. hypertension, diabetes, hyperlipidaemia) are recognised as risks for both VCID and AD with salt intake, chronic inflammation and gut infection now emerging as additional risk factors (7, 8) .
- Thus it is proposed that understanding the earliest events leading to white matter changes could provide vital opportunities to prevent brain damage at the earliest stages and ameliorate its impact on cognitive decline and precipitation of dementia (23) .
- The authors focus on rodent models which have been developed and refined over the last few years to mimic the chronic hypoperfusive state in VCID and which are used as a basis to probe mechanisms related to reduced perfusion of the brain.
Cerebral blood flow alterations in models
- In order to study early pathological events that may lead to VCID, rodent models of chronic cerebral hypoperfusion were first established using occlusion or ligation of both common carotid arteries in rats (2 vessel occlusion) (see review 28).
- A refinement of rat models of cerebral hypoperfusion was introduced to more faithfully represent the subtle reductions in flow in VCI.
- Bilateral common carotid artery stenosis (BCAS), by application of microcoils, reduces luminal diameter to approximately 50% in young adult C57Bl/6J mice (36) .
- Thus, the studies emphasise the utility of arterial spin labelling as a quantitative and non-invasive tool for assessing global and regional blood flow changes in models of hypoperfusion.
- In another model an ameroid constrictor is applied to the right common carotid artery resulting in gradual occlusion of the vessel over 28 days, whereas placement of a microcoil to the left common carotid artery induces ∼50% arterial stenosis (43) .
Parenchymal alterations
- Rat 2 vessel occlusion models have been extensively studied since they were first found to develop white matter rarefaction similar to that in humans (28) .
- Paranodal septate-like junctions are damaged and axon-glial integrity is disrupted, as determined by spatial distribution of myelin-associated glycoprotein staining (46) .
- In response to increasing durations of hypoperfusion, microglia gradually augment in parallel with the evolving damage to myelinated axons, resulting in a marked and sustained increase in microglial number, particularly in the white matter (36, 37, (45) (46) (47) .
- Astrogliosis can also be observed but these changes appear to occur later than microglial alterations in BCAS models with diffuse white matter injury (48) .
- Pathological changes in the gradual stenosis models are similar to the microcoil model but as expected these progress more slowly.
Small vessel and BBB changes
- The BBB changes appear less prominent (51) .
- Other studies, in which white matter damage appears more severe, showed earlier BBB disruption at 3 and 7 days after BCAS (52) .
- A systematic study of the BBB, including tight junction proteins, across a range of times post-hypoperfusion in the different models is required to assess the dynamics of the BBB and whether these changes may be transient or sustained.
- Sustained hypoperfusion can also induce morphological small vessel changes such as increased thickening and fibrosis of capillary walls, which are one of the characteristic features of human SVD (2).
- In the gradual stenosis models, small vessel and BBB changes are yet to be described.
Brain atrophy
- Clinical imaging studies have demonstrated both whole brain and regional brain atrophy in VCID (54, 55) .
- Brain atrophy, in particular medial temporal lobe atrophy and subcortical atrophy, is associated with cognitive decline and can potentiate the effect of white matter lesions on cognition (54, 56) .
- Nishio et al. (58) reported no apparent change in cortex or corpus callosum at 8 months following BCAS surgery, however the hippocampal volume was found to be significantly reduced in hypoperfused mice.
- In agreement with this finding, hippocampal glucose uptake was also reduced when assessed with 18 F-FDG PET.
- Together, these findings suggest that brain atrophy occurs at later time points following hypoperfusion, and are secondary to neuronal loss and white matter damage.
Cognitive impairments
- It was proposed many years ago that chronic cerebral hypoperfusion leads to cognitive impairment (59) but human studies have shown at best a moderate association.
- Interestingly, poor performance in the odour discrimination task suggests that cognitive functions related to olfaction are also impaired (64) , which is also seen as an early sign of cognitive impairment in patients with various neurodegenerative diseases, indicating the relevance of this model to human VCID.
- The development of the BCAS mouse model demonstrated that chronic cerebral hypoperfusion causes deficits mainly in spatial working memory (45, 65-67) using a conventional 8-arm radial maze or Y-maze tests.
- The authors highlighted that 1 month after BCAS, spatial working memory is impaired while reference memory remains intact, probably due to the select disruption of frontal-subcortical circuits (45) .
- The emergence of deficits in both working and reference memory likely reflects the presence of white and grey matter pathology including whole brain and hippocampal atrophy (48, 70) .
Mechanisms of the hypoperfusion models
- As indicated above, rodent models of hypoperfusion recapitulate some pathological features observed in VCID, such as disruption of white matter integrity, microvascular alterations and atrophy.
- These alterations are related to cognitive deficits.
- The precise molecular and cellular mechanisms that lead to such changes are currently being unravelled as outlined below:.
Hypoxia-induced white matter damage
- Following vessel occlusion or carotid stenosis, cerebral perfusion is demonstrably reduced (28, 37) but it is less clear whether these changes affect tissue oxygen tension and whether there are differences between white matter and grey matter to account for their differential vulnerability.
- Levels of pO2 are profoundly decreased in the corpus callosum at 3 days, 1 week and 6 weeks following BCAS surgery to levels consistent with hypoxic conditions.
- Hypoxia, in addition to deleterious effects on oligodendrocytes, can also directly induce changes in BBB permeability (76) .
- In the aged primate brain, microglial reactions predominate in the white matter and correlate with cognitive impairment (83) .
- Higher numbers of microglial cells correlate with decreased nodal gap length and increased paranodal axon-glial disruption, supporting the idea that the structural alterations observed in response to hypoperfusion could be secondary to a pro-inflammatory environment.
Microvascular inflammation
- Endothelial dysfunction is considered to be one of the pivotal mechanisms of the structural and functional cerebral vessel alterations in SVD (10, 11) leading to VCID.
- Cerebral hypoperfusion upregulates the expression of adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), markers of endothelial cell activation (68, 86) .
- Increased MMPs have been consistently shown in hypoperfusion models (90) (91) (92) and increased expression has been shown in the white matter (91, 92) localised to microglia and the endothelium (91) .
- MMPs are proteases that degrade the extracellular matrix as well as tight junctions between endothelial cells and have been implicated in BBB breakdown in neurodegenerative diseases (52) .
- Additionally, age, a key risk factor for VCID, is also associated with alterations in microglial phenotype and function (99) .
Neuro-glio-vascular function
- Instead, disruption of the finely tuned interplay between cells of the neuro-glio-vascular unit (oligodendrocytes, endothelial cells, astrocytes and end-feet contacts, pericytes, microglia and neurons) (for review see 101) likely contributes to the pathophysiology.
- Astrocytic activation has been detected in white matter at 7 days following BCAS surgery in mice (107) and has the potential to disrupt the contact between astrocytes and blood vessels.
- Thus, the cellular interactions within the neuro-glio-vascular unit may be critically important for maintaining tissue health.
- Taken together, hypoperfusion is likely to drive key pathways related to hypoxia, inflammation and BBB disruption, resulting in progressive deterioration of the neuro-gliovascular unit .
Interplay between cerebral hypoperfusion and co-morbidities
- Post-mortem studies in brains from the elderly indicate the presence of a variety of pathological lesions or mixed pathologies (such as white matter changes, infarcts, microbleeds and amyloid pathology) (115) .
- This would be important in identifying drug targets and as a basis for testing treatments.
- White matter lesions were also exacerbated in the presence of occlusion and dietary factors (116) .
- Since rodents do not naturally accumulate A most studies use transgenic models that harbour mutations in the amyloid precursor protein associated with rare familial forms of AD .
- Thus, conceivably there may be an interplay between increased NOX and amyloid following hypoperfusion that exacerbates neurovascular dysfunction.
Pre-clinical investigation of drug targets
- Assessment of pipeline drugs can be evaluated in preclinical models before clinical trials are initiated providing a translational opportunity.
- In the BCAS model, pretreatment and post-treatment of cilostazol reduced endothelial activation, suppressed microglial proliferation and improved cognitive function without affecting resting CBF and white matter integrity (68) .
- Other studies, in which white matter is damaged by cerebral hypoperfusion via unilateral occlusion (143) , two vessel occlusion (144) and in hypertensive stroke prone rats with two vessel occlusion (141) have also convincingly shown protective effects of minocycline.
- A broad spectrum MMP inhibitor was also shown to protect against BBB opening in the mouse BCAS model and against the ensuing white matter pathology and cognitive deficits (52) .
- Therefore, the implementation of even limited environmental enrichment may be beneficial for patients diagnosed with VCID.
Important considerations of the models; How can we close the gap between rodent models of chronic cerebral hypoperfusion and human VCID?
- In order to effectively translate information generated from animal models such as cerebral hypoperfusion, the authors need to consider the general design, methodology and reporting of preclinical animal studies.
- Additionally, monitoring blood flow alterations in the white matter would be important.
- In some studies animals are allowed to recover between the placement of the first and second coils to allow restoration of cerebral haemodynamics (40, 44, (46) (47) (48) .
- Application of microcoils or ligation of vessels may alter autoregulation, vascular stiffness or pulsatility and impact on the dynamics of cerebrospinal fluid circulation.
- Several rodent models reflect aspects of human VCID and are pertinent to tease out specific questions that are impossible to readily address in human studies.
Figure legends:
- Figure 1 Multiple experimental models have been developed in order to study disruption of cerebral blood flow.
- Due to the severe reductions in flow typically observed in models of ischaemia and resultant ischaemic neuronal damage, new models have since been developed and refined in order to mimic the subtle yet chronic reductions in blood flow relevant to vascular cognitive impairment.
- The levels of cortical surface cerebral blood flow (CBF) estimates at indicated time points (before, and 1, 3, 7, 14, and 28 days after each surgery) are shown as percentage of the baseline CBF.
- B. At all times post-hypoperfusion, pO2 levels were significantly reduced to hypoxic levels (<10mmHg) ****p<0.0001 Figure 4 (B) Representative responses to whisker stimulation from sham and hypoperfused mice.
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Citations
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139 citations
Cites background from "Chronic cerebral hypoperfusion: a k..."
...Contribution of laboratory animal studies Several animal models have been developed to gain mechanistic insights into the pathophysiology of the WM matter and enable translation to the clinic (Duncombe et al. 2017)....
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Cites result from "Chronic cerebral hypoperfusion: a k..."
...While similar conclusions may be made for GLUT-1 results as those for COL4, these novel findings with respect to dementia suggest there are compensatory mechanisms in the WM to maintain reserves of blood flow within capillaries and ameliorate cerebral hypoperfusion [11, 16]....
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54 citations
Cites background from "Chronic cerebral hypoperfusion: a k..."
...Cerebral hypoperfusion up-regulates the expression of adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), markers of endothelial cell activation [80]....
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...It is not yet known whether inflammation is a primary driver of VCID and whether this neuroinflammation is triggered by intrinsic or systemic processes [80]....
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References
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Frequently Asked Questions (16)
Q2. What is the role of MMPs in neurodegenerative diseases?
MMPs are proteases that degrade the extracellular matrix as well as tight junctions between endothelial cells and have been implicated in BBB breakdown in neurodegenerative diseases (52).
Q3. What is the importance of monitoring blood flow in the preclinical animal studies?
Since the basis of the models are dependent on the extent of reduction of cerebral perfusion, it would be critical to monitor blood flow in each study.
Q4. What is the role of astrocytes in the repair of white matter damage?
Astrocytes have also been shown to support oligodendrogenesis through secretion of brain derived nerve growth factor (BDNF) in order to promote repair of white matter damage following BCAS in mice (110).
Q5. How long after BCAS did the brain develop a deficit in spatial working memory?
After long-term i.e. 6 months of hypoperfusion after BCAS, both spatial working memory and spatial reference memory were impaired (48).
Q6. How is the BBB disrupted in the gradual stenosis model?
In the rat 2 vessel occlusion model, BBB disruption is observed as early as 3 hours postocclusion most likely as a result of the sharp and severe CBF reduction in this model (51).
Q7. What is the method for assessing regional alterations in blood flow?
MRI with arterial spin labelling or similarly sensitive methods would be ideal to assess regional alterations in blood flow, particularly in subcortical areas.
Q8. How does BCAS affect blood flow in young adult C57Bl/6J mice?
Bilateral common carotid artery stenosis (BCAS), by application of microcoils, reduces luminal diameter to approximately 50% in young adult C57Bl/6J mice (36).
Q9. What are the main factors that need to be considered in the preclinical testing of future drug?
Age and additional co-morbidities (such as systemic inflammation) need to be carefully factored in to preclinical testing of future drug targets if the authors are to enable meaningful translation from models to the clinic.
Q10. How long does it take to recover blood flow in young mice?
With increasing time there is a recovery of blood flow in young mice to 15-20% baseline levels at 1 month when measured by laser Doppler ultrasound or laser speckle imaging (36, 37).
Q11. What did Weaver et al. (116) demonstrate?
Weaver et al. (116) demonstrated the utility of electron paramagnetic resonance oximetry to study white matter pO2 reductions longitudinally in a mixed SHRSP/Japanese Permissive Diet model with unilateral common carotid artery occlusion.
Q12. What is the need to provide mechanistic insight of white matter changes through the development of animal?
In addition to correlative pathological and imaging studies in human, there is a need to provide mechanistic insight of white matter changes through the development of relevant animal models and translate these findings to the clinic (24).
Q13. How did Nishio et al. (58) find the hippocampal?
Nishio et al. (58) reported no apparent change in cortex or corpus callosum at 8 months following BCAS surgery, however the hippocampal volume was found to be significantly reduced in hypoperfused mice.
Q14. What is the link between impaired neurovascular coupling and white matter lesion development?
a direct causal link between impaired neurovascular coupling and white matter lesion development has yet to be proven, as disrupted neurovascular coupling may reflect reduced tissue metabolic demand as a result of other ongoing pathological processes.
Q15. How did the occlusion model of chronic cerebral hypoperfusion work?
In order to study early pathological events that may lead to VCID, rodent models of chronic cerebral hypoperfusion were first established using occlusion or ligation of both common carotid arteries in rats (2 vessel occlusion) (see review 28).
Q16. In what model of CAA did Okamoto et al. (120) show?
In another TgAPP model of CAA, Okamoto et al. (120) showed that blood flow reductions at 12 weeks following BCAS were greater in TgAPP than in wild type mice.