Cerebral hypoperfusion is one of the causes that contribute to the occurrence and development of VCI and dementia. With the decrease in cerebral blood flow (CBF), the cognitive function of patients also deteriorates16,17. This reduction in CBF is related to the severity of WMHs under T2 sequences of MRI18,19. However, the pathological mechanisms of hypoperfusion-induced white matter damage remain incompletely elucidated, and there are still no effective diagnostic methods to predict the occurrence of WMH. In the present study, we investigated the expression of MATK in peripheral blood and verified whether MATK could be a biomarker for predicting and diagnosing WMH and VCI.
Studies have shown that CCH, like cerebral small-vessel disease (CSVD), was often ignored by patients due to less severe clinical symptoms or lower mortality than common stroke. With the development of neuroimaging equipment, MRI can allow us to detect the presence of CSVD at an early stage. Similarly, we can prevent the progression of CCH through effective treatment measures. MRI examination in elderly patients with cognitive dysfunction shows that WMH is one of the biomarkers for the diagnosis of CSVD20–22. MRI scans of 560 older individuals, the subjects were followed up with annual neuropsychological examinations for 3 years and evaluation of instrumental activities of daily living for 7 years, found that WMH was an independent predictor of cognitive impairment in a multivariate linear mixed model 23. However, just relying on WMH is not specific for the diagnosis of CSVD, because it is difficult to distinguish CSVD or CCH from other demyelinating diseases such as multiple sclerosis and optic neuromyelitis by WMHs24. There was also some mismatch between WMH and cognitive function scores in patients with CSVD. Among 156 patients with CSVD, 37 (23.7%) showed mismatching. Conventional imaging features and penetrating artery injury may explain this heterogeneity25. Therefore, the use of other biomarkers to further clarify the prognosis of CCH is particularly important. In patients with a clinical diagnosis of CSVD, changes in several biomarkers such as low molecular weight neurofilament(NF-L), matrix metalloproteinase-9(MMP-9), tissue inhibitor of metalloproteinase-1(TIMP-1), matrix metalloproteinase-2 (MMP-2) and albumin CSF/plasma ratio were found to be increased26–28. In our study, we found that the relative mRNA expression of MATK in the peripheral blood of patients with CCH was significantly decreased, and it was significantly negatively correlated with Fazekas scales. Logistic regression analysis indicated that MATK and MoCA were two independent factors strongly associated with WMH. The area under the ROC curve of MATK was 0.743, which was not significantly different from the final regression model 0.803, suggesting that MATK can be used as a biomarker for diagnosis and prediction of WMH, indicating that MATK can provide help in the diagnosis and prognosis of CCH.
MATK is mainly expressed in blood cells and the brain [12]. It has been reported that IL-4 can induce MATK/CHK expression in human peripheral blood mononuclear cells, while IFN-γ can inhibit it13. Another study showed that mouse primary microglia cells were stimulated with IL-4 and IFN-γ respectively to obtain differential gene expression profiles of microglia cells in vitro. The expression of the MATK molecule was significantly up-regulated under IL-4 (M2 polarization) stimulation, but significantly down-regulated under IFN-γ (M1 polarization) stimulation29. These studies suggest that MATK in myeloid cells has the potential to inhibit inflammation both in the peripheral circulation and the central nervous system, with its expression significantly increased in the anti-inflammatory environment.
Microglia cells are also one of the mechanisms that cause WMH in the brain of patients with CSVD. As resident macrophages of the central nervous system (CNS), microglia cells play the role of endogenous innate immunity. Although microglia cells are generally favorable initially in performing immune functions, prolonged or excessive activation can lead to cytotoxic effects30,31. When chronic hypoperfusion and blood-brain barrier destruction occur in the brain, the infiltration of peripheral mononuclear macrophages and overactivation of microglia will aggravate the injury of white matter and cause the demyelination of axons. This is one of the important mechanisms of CCH brain injury.
In one early study, the degree of microglial activation increased with aging, becoming diffusely distributed within the white matter. The degree of activation correlates with the aggravation of cognitive impairment32. A large number of microglia were activated in the acute or progressive stages of multiple sclerosis in the white matter lesion area, presenting an IFN-γ activation phenotype, which changed to IL-4 activation in the disease remission phase. Macrophages in the peripheral blood also further increase this process after the breakdown of the blood-brain barrier. Simultaneous removal of microglia in vivo also alleviates blood-brain barrier disruption in EAE, and this mechanism could be modulated by altering microRNA-126 expression in microglia33.
In our study, we found that MATK expression was significantly decreased in the peripheral blood of patients with severe degrees of cerebral WMHs, suggesting that reduced MATK can drive macrophages to the IFN-γ phenotype in the peripheral blood, which has an inflammatory damaging function. While within the brain, it may also promote microglia towards a pro-inflammatory phenotype to damage the white matter. But the role of MATK on the phenotypic conversion of microglia still needs further studies to be clarified.
WMH is very common in older adults and develops with age 34. Black race, female sex, and the presence of the APOE-4 allele were all associated with greater WMH burden or progression34,35. These factors are all non-modifiable factors as in hereditary CSVD, while the risk factors for WMH severity and progression are also associated with cerebrovascular, cardiometabolic, and nutritional34. Of these, hypertension had the strongest association. In a cross-sectional study, increased BP was clearly associated with the presence or severity of WMH. Hypertension is commonly found to be associated with the subsequent appearance of WMH in studies of early hypertension, with elevated blood pressure being able to increase the risk of developing WMH after 5 and 20 years. Similarly, midlife and older age BP were both associated with an increased risk of WMH36.In another study, increasing midlife BP was significantly associated with WMH volume in old age37. A meta-analysis of the effects of antihypertensive drugs on CSVD showed that patients in the intensive BP-lowering group had significantly less WMH progression38.
The limitations of this study are as follows. First of all, due to the limited number of cases collected, there may be some selective bias. It is necessary to expand the sample size in later studies to observe the overall change of MATK expression in CCH patients. Secondly, the WMH patients included those with or without MCA/ICA artery stenosis. Intracranial artery stenosis itself is also a risk factor for WMH. The role of MATK in the occurrence of WMH can be determined by observing whether intracranial artery stenosis will cause changes in MATK expression alone. Third, MATK in peripheral blood is derived from monocytes and cannot fully represent the expression of MATK in intracranial microglia. Further animal experiments are needed to clarify the changes of MATK in the peripheral and brain pathology of WMH. As there are currently limited drugs that can treat WMH caused by CCH, the decreased expression of MATK in peripheral blood found in our study is a biomarker in CCH patients, so MATK may become a potential target for treating WMH in patients with CCH. Finding drugs that act on the MATK signaling pathway may be an effective way to treat the progression of WMH.