Atherosclerosis is a major cause of morbidity and mortality in Western societies (1,2). It is a chronic inflammatory disease of the arterial wall, which is characterized by lipid accumulation, leukocyte infiltration and smooth muscle cell proliferation. Atherosclerosis is considered to be a risk factor for cognitive deterioration in the elderly, including Alzheimer's disease (AD) (3,4). The latter is a neurodegenerative disease, which is characterized by dementia, along with a dense deposition of amyloid-β (Aβ) protein in senile plaques in the brain, hyperphosphorylated tau protein and neuron loss. Several findings suggest that atherosclerosis and AD are linked: i) Atherosclerotic vascular disease and AD share common risk factors, such as hypertension, diabetes, hypercholesterolemia, and apolipoprotein ε4 allele (5). In addition, old age is a major risk factor for both atherosclerosis and AD (6). ii) There are reports of a correlation between carotid atherosclerosis and AD (7) as well as atherosclerosis of the circle of Willis and AD (8,9). Moreover, coronary artery disease is increased in AD patients (10). iii) In a transgenic mouse model of AD (B6Tg2576), early atherosclerosis lesions were detected and were positively correlated with cerebral β amyloid deposits when mice were fed a normal diet (11) or atherogenic diets (12). iv) Brain cholesterol affects the Aβ formation from amyloid precursor protein (APP) (13,14). v) APP and Aβ are present in human carotid plaques (15). vi) APP transgenic mice with apolipoprotein E (ApoE) deficiency had increased atherosclerosis and vascular inflammation (16). However, whether atherosclerosis contributes to AD or they just share similar epidemiology is still not known. ApoE is the main lipid carrier protein in the brain, and it is released by astrocytes in order to supply neurons with cholesterol. The human ApoE gene has 3 allelic variants (ε2, ε3, and ε4). The ε4 allele has been associated with a higher risk of cardiovascular disease and AD, while the ε2 allele has a protective effect against AD. In mice, the association between ApoE deficiency and AD is not clear (17–26). Recent findings from our group and others demonstrate the involvement of coagulation factors in both atherosclerosis (27,28) and AD (29,30). The contact activation pathway plays an essential role in hemostasis, and also in the progression of thrombosis and inflammation (31). In addition, factor XII, the initiator of contact activation, has been shown to be activated by aggregated Aβ (30), which resulted in activation of factor XI (FXI), thus enhancing brain thrombin generation. In humans, increased FXI is associated with increased incidence of ischemic stroke (32), whereas subjects with severe FXI deficiency have a reduced incidence of ischemic stroke (33). High levels of thrombin were found in the circulation, brain parenchyma and micro-vessel walls of AD patients and also in AD mouse models (30,34,35). In addition, high levels of fibrinogen are correlated with increased cerebral amyloid angiopathy and microglial activation when assessed in AD mouse models (36). Procoagulant proteins, including FXI, were found adjacent to macrophages and smooth muscle cells inside atherosclerotic plaques (27,37). Deprivation of FXI in ApoE knockout (KO) mice aged 24 and 42 weeks resulted in a significant reduction of atherosclerosis in the aortic sinus and aortic arch in comparison to ApoE KO mice (28). Targeting FXI prevented thrombosis on acutely ruptured atherosclerotic plaques (38). Furthermore, APP and Aβ were found in advanced human carotid plaques, in proximity to activated macrophages and platelets (15). Hence, it can be suggested that coagulation factors, including FXI, play a role in both atherosclerosis and AD. Old age is the main risk factor for AD; therefore, in this work, we sought to study whether FXI deficiency in elderly ApoE KO mice would decrease pathological changes compatible with atherosclerosis and AD.

Previous studies have suggested that the coagulation pathway is involved in both atherosclerosis and AD (28,30). This motivated us to investigate the influence of FXI deficiency on atherosclerosis and pathological brain changes associated with AD in very old ApoE KO mice. Our results show that: I. although elderly ApoE KO mice develop advanced aortic atherosclerotic lesions, they do not develop senile Aβ plaques in the brain, and II. FXI deficiency reduces the aortic atherosclerotic burden, and similar to ApoE KO mice, ApoE/factor XI double KO (ApoE/FXI DKO) mice do not develop Aβ plaques.

We first asked whether FXI deficiency would inhibit atherogenesis in very old mice, similar to its effects in younger mice (28). The results show that elderly mice develop advanced atherosclerosis, and 30% of the aortic arch is covered with atherosclerotic lesions. This is in accord with previous reports showing the development of advanced atherosclerotic lesions in ApoE KO mice that have been fed a normal diet (39). FXI deficiency reduced the lesion area by 33%, similar to its effect in 42-week-old ApoE KO mice (28). Congenital FXI deficiency or targeting FXI do not cause spontaneous bleeding (40–42), in contrast to targeting factor VIII or factor IX. Furthermore, it was recently shown that inhibition of FXI synthesis by antisense oligonucleotide reduces blood pressure in mice and rats (43). Therefore, it is conceivable to use FXI KO mice to study the effect of the coagulation pathway on pathological changes in the brain in elderly mice.

As we used very old ApoE KO mice, we anticipated that these mice would develop pathological changes compatible with AD, including senile Aβ plaques. Unexpectedly, no lesions compatible with AD (i.e., Aβ plaques, brain atrophy, increased glial cells or increased levels of cortex inflammatory cytokines, such as IL-6 and TNF-α) were detected in the ApoE KO mice or in the FXI-deficient mice. There could be several explanations for such findings, which may all relate to ApoE's role in the brain. In contrast to ApoE4 transgenic mice that clearly develop the typical symptoms of AD, the effect of ApoE deficiency on pathological changes compatible with AD is still unknown. Several studies show that ApoE KO mice are associated with an increased risk of developing AD-related pathologies, that is, memory deficit (20), tau phosphorylation (19,26), a leaky blood-brain barrier (BBB) (24), higher levels of protein oxidation (18), age-dependent synaptic loss (23) and even Aβ deposits (26). In addition, a recent study shows synaptic loss and dysfunction in mice that express ApoE in peripheral tissues, but that have severely reduced ApoE in the brain (22). Notably, no pathologic brain changes were observed in 9-month-old ApoE-deficient mice (25). Bales et al (17) claimed that the ApoE KO mouse is not a suitable model to study AD, since ApoE facilitates Aβ deposition, while completely ablation of ApoE, decreased cerebral Aβ sedimentation. It is important to note that Bales et al demonstrated their findings in mice aged 6–11 months and studied only Aβ deposition in the brain, while we studied 15-month-old mice and also investigated atherosclerosis-related pathologies in the brain. Taken together, ApoE KO and ApoE/FXI DKO cannot serve as a model to study AD or pathologic brain changes related to atherosclerosis. It appears that there is a dichotomy between the brain and peripheral blood vessels. The mechanism by which ApoE KO protects against brain pathology should be further studied as it may prove helpful for future treatment for senile dementia. Yet, targeting FXI can still serve as an important therapy to attenuate atherosclerosis. As such, it is conceivably that elderly patients might benefit from FXI target therapy to reduce the process of peripheral atherosclerosis, though more studies are needed.