While there was no direct relationship of serum LDL-C with brain Aβ and tau deposition, we found significant LDL-C x brain Aβ synergistic interaction on tau deposition, which supported our hypothesis that blood LDL-C moderates the association between cerebral Aβ and tau deposition. Subsequent subgroup analyses demonstrated that the positive association between Aβ and tau deposition was stronger at higher LDL-C levels than at lower LDL-C levels. In contrast to LDL-C, none of the other lipids demonstrated a moderating effect on the Aβ-tau association, nor did they show a direct association with Aβ or tau deposition.
The finding that higher LDL-C levels strengthened the association of brain Aβ with tau deposition is generally consistent with previous clinical reports on the relationship between blood LDL-C and AD-related cognitive decline.(3, 4) This finding also aligns with a report showing a reduced risk of AD dementia in users of statin drugs that lower blood LDL-C.(30) Similarly, the association between statin use and reduced burden of neurofibrillary tangles at autopsy was reported.(31) Although it is not easy to provide the exact mechanisms which underlie the moderation effect of LDL-C on the Aβ-tau relationship, some possible explanation can be made. As the brain is the most cholesterol-rich organ in the body, containing about 20% of the body's total cholesterol,(32) changes in cholesterol levels can lead to brain pathology.(33) However, the presence of the blood-brain barrier (BBB) prevents blood cholesterol from entering the brain.(34) Nevertheless, free radicals, formed under the influence of high blood cholesterol, can destroy the BBB and, as a result, increase brain cholesterol.(34, 35) Elevated brain LDL-C levels have been shown to promote neuroinflammatory responses.(36) The induced neuroinflammatory response in turn influence tau pathogenesis.(37) A study using an animal model reported that intra-cerebral administration of a potent inflammatory substance to myeloid receptor promoted tau hyper-phosphorylation and tangle formation.(38) It was also reported that minocycline treatment reduced cortical tau phosphorylation through reduced inflammation in a mouse model of tauopathy.(39) Other transgenic AD model study showed that inflammation exacerbates tau pathology by a cyclin-dependent kinase 5-mediated pathway.(40) Meanwhile, microglia, an important regulators of neuroinflammation, attaches to amyloid plaques and abundantly release cytokines that induce neuroinflammation, which potentially exacerbate tau pathology in the periphery of the amyloid plaques.(41, 42) Given all together, increased blood LDL-C may aggravate tau accumulation closely related with amyloid pathology(10–12) by elevating brain LDL-C and in turn promoting neuroinflammatory response.
In contrast to the moderation effect of LDL-C, other lipids did not have any relationship with Aβ or tau accumulation in our study. Several studies have shown that high TC or low HDL-C levels are associated with clinical diagnosis of AD dementia or cognitive impairment.(43, 44) However, direct comparison between the findings of the previous studies and those of the current study is not easy because the considerable differences in study methodology. While we focused on the associations of lipids on in vivo AD pathologies, most previous studies did not measure the pathologies and just investigated the relationship with clinically defined AD dementia or cognitive impairment.(43, 44) Approximately 14–32% of clinically defined AD dementia cases(45) and 29–73% of MCI cases did not exhibit Aβ pathology in the brain.(46) In addition, many of the previous studies investigated the relationship between lipid levels in midlife and dementia in late-life,(43, 44) while we measured lipid levels and in vivo brain pathologies in late-life. A study reported that high midlife TG levels were associated with increased brain Aβ and tau pathology after 20 years.(13)
Our exploratory analyses showed that serum LDL-C was positively associated WMH volume, but other lipids were not. This is consistent with the results of other studies: An observational study showed that high blood LDL-C was associated with increased WMHs,(47) and another study also reported that LDL-C was related with periventricular WMHs, but other lipids, i.e., TC, HDL-C, and TG, were not.(48) Since brain endothelial cells are sensitive to circulating LDL-C levels, impaired vascular responses are induced through oxidative stress and the secretion of inflammatory mediators caused by increased LDL-C.(49)
Our result that serum LDL-C has a moderating influence on brain Aβ-tau association in human is new. The present study does, however, include several limitations that need to be taken into account. Firstly, because this research was cross-sectional, no causal connections can be determined by the results. Additional prospective longitudinal research is necessary. Second, it may have been difficult to identify a direct link between higher serum LDL-C levels and each of Aβ and tau deposition because of the limited number of participants (n = 48, 34.6%) with unusually high serum LDL-C levels (116mg/dL). Lastly, while amyloid PET and MRI scans were performed at baseline, tau PET scans were conducted on average 2.6 years (standard deviation = 0.3 years) after the baseline visit. The relationship between blood lipids and tau deposition may have been affected by this time gap. However, the results remained consistent when the temporal gap was adjusted as an extra covariate.