Blood Pressure variability and microvascular dysfunction
A new mechanism was discovered, the capillary-to-arteriole electrical signalling. Kir2.1 channels sense neuronal activity and initiating electrical vasodilatory signal. This mechanism is impaired in hypertensive mouse models as well as in CADASIL mice suggesting that Kir2.1dysfunction is a common target of SVDs. It was shown that phosphatidylinositol 4,5-bisphosphate (PIP2), an activator of Kir2.1 channel activity, is the cause of decreased Kir current. PIP2 and Kir2.1 could function as possible novel therapeutic targets.
Clinical trials showed a relationship between Blood Pressure (BP), BP-Variability and stroke risk. Lacunar stroke is strongly associated with recent increases in BP, but MRI markers of SVD with historic BP. BPV from beat to beat predicts recurrent events and might be a novel risk marker. Increased BPV is associated with increased stiffness of the large arteries. Cerebral pulsatility index is associated with global SVD burden and treatments targeting pulsatility may have the potential to prevent SVDs. We showed that single BP measurements in the clinic are not as reliable as Home Blood Pressure Monitoring because they could mask hypertension.
Blood brain barrier (BBB) and perivascular flow
We have not identified any signs of BBB defects in animal models. Detoriation of the glymphatic system, important for the clearance of waste products from the brain, plays a key role in initiation and progression of SVD. We could show that influx and clearance of glymphatic tracers is enhanced in CADASIL and reduced in sporadic SVDs, suggesting that the activity of the glymphatic system is modified in opposite directions.
INVESTIGATE-SVDs assessed brain vascular functions, BBB, cerebrovascular reactivity (CVR) and vascular pulsatility, in the same patient, which is truly ground breaking. We showed that BBB leak predates tissue destruction and lacune formation. Patients with higher White Matter Hyperintensities volume tended to have higher BBB leakage and arterial pulsatility and lower CVR. In CADASIL vs sporadic SVDs BBB leakage seems to be lower and CVR tended to be higher, despite higher WMH burden. We did not find strong associations between the different vascular dysfunction metrics; all three were abnormal and related to the severity of SVD but in different proportions. Our position statement on optimal methods for assessing BBB leak is widely cited.
Microvascular matrisome
We selected two proteins TIMP3 and COL4A1 which are deregulated in the brain matrisome. Excess of TIMP3 impairs functional hyperemia by upregulation of KV channels in arteriolar SMCs and downregulation of Kir2.1 channel in capillary endothelial cells. This novel mechanism establishes an important paradigm for cerebrovascular regulation. A novel segment between arterioles and capillaries, the transitional segment (TS), was identified. Col4a1 mutant mice with downregulated COL4A1 exhibit hypermuscularisation of the TS and a loss of SMC in arterioles. This raises the pressure in the arteriole and promote rupture at the site of SMC loss. Hypermuscularization of TS was confirmed in humans, representing a general mechanism of genetic and sporadic intracerebral haemorrhage.
Using 7T MRI technology, ZOOM@SVDs showed for the first time, that several aspects of small vessel function are affected in patients with SVDs. Functional abnormalities appear to differential affect different vessel populations, are related to disease severity, and may have different signatures in different forms of SVDs. With this we have established novel markers of microvascular malfunction in patients with SVDs that will help to further unravel the aetiology and may be used as outcome markers to show small vessel target engagement for novel treatments.
Inflammatory mechanisms
We found resident microglia activation in mouse SVD models. Depletion of microglial improved cognitive impairment supporting the hypothesis that microglia play a critical role in pathogenesis of cognitive impairment.
MRI guided autopsy specimens showed activated microglia and extravascular fibrinogen without significant peripheral immune cell activation. Flow cytometry on samples from all three clinical studies showed a shift in percentage of classical monocytes that was associated with SVD MRI markers. Similarities between the human and mouse studies validate our results of the involvement of the innate immune system in pathogenesis. Future studies investigating whether monocyte populations can act as a potential biomarker for disease severity/progression are needed.
Validation of mechanisms through intervention
In hypertensive SVD mouse model, treatment of amlodipine, an antihypertensive drug classified as a voltage-dependent calcium-channel blocker, prevented the development of functional hyperemia deficits. Despite similar efficacy in reducing blood pressure, losartan, an angiotensin II type 1 receptor blocker, did not exhibit protective effects on functional hyperemia. This drug class effect was attributed to losartan-induced elevated aldosterone levels that have a negative impact on functional hyperemia. In this context, hyperaldesteronemia was discovered as a new hidden risk factor.
In the multicentre trial TREAT-SVDs three antihypertensive medications are compared with respect to their beneficial effects on microvascular function in patients with SVDs. Recruitment of Patients with sporadic SVDs is completed. Initial analysis revealed that patients with sporadic SVDs show a different vascular risk profile than CADASIL patients. This was expected since CADASIL affects younger patients.
