Loss of Endothelial Nitric Oxide Synthase Promotes p25 Generation and Tau Phosphorylation in a Murine Model of Alzheimer’s Disease

Abstract

Rationale Alzheimer’s disease (AD) has an unknown etiology; however, cardiovascular risk factors are associated with a higher incidence of AD. A defining feature of endothelial dysfunction induced by cardiovascular risk factors is reduced bioavailable endothelial nitric oxide (NO). We previously demonstrated endothelial NO acts as an important signaling molecule in neuronal tissue. Objective We sought to determine the relationship between the loss of endothelial nitric oxide synthase (eNOS) and tau phosphorylation in neuronal tissue. Methods and Results We utilized eNOS knockout (−/−) mice as well as an AD mouse model, APP/PS1 that lacked eNOS (APP/PS1/eNOS−/−) to examine expression of tau kinases and tau phosphorylation. Brain tissue from eNOS−/− mice had statistically higher ratios of p25/p35, indicative of increased cyclin-dependent kinase 5 (Cdk5) activity as compared to wild type (n=8, P<0.05). However, tau phosphorylation was unchanged in eNOS−/− mice (P>0.05). Next, we determined the role of NO in tau pathology in APP/PS1/eNOS−/−. These mice had significantly higher levels of p25, a higher p25/p35 ratio (n=12–14, P<0.05) and significantly higher Cdk5 activity (n=4, P<0.001). Importantly, APP/PS1/eNOS−/− mice also had significantly increased tau phosphorylation (n=4–6, P<0.05). No other changes in amyloid pathology, antioxidant pathways, or neuroinflammation were observed in APP/PS1/eNOS−/− mice as compared to APP/PS1 mice. Conclusions Our data suggests that loss of endothelial NO plays an important role in the generation of p25 and resulting tau phosphorylation in neuronal tissue. These findings provide important new insights into the molecular mechanisms linking endothelial dysfunction with the pathogenesis of AD.

Untangling Neurons With Endothelial Nitric Oxide

“…Lack of eNOS is not sufficient to induce ptau, since eNOS-null mice did not exhibit evidence of tau hyperphosphorylation despite the p25 increase, whereas lack of eNOS was able to increase ptau in APP/PS1 mice 3 . This observation is consistent with the notion that Aβ fosters tau phosphorylation and, presumably, neurotoxicity.…”

“…Tau phosphorylation has been linked to misfolding and aggregation, but its role in neurotoxicity is complex and depends on specific phosphorylation sites 16 . The observation by Austin et al 3 that eNOS deletion does not lead to neurofibrillary tangles in APP/PS1 mice does not rule out a potential effect of NO deficiency on tau-induced cognitive dysfunction. This is because in animal models as in humans neurofibrillary tangles are not required for the cognitive dysfunction associated with tau 16 .…”

“…Austin et al 3 discovered that eNOS is critical for the maintenance of this delicate balance. By crossing mice lacking eNOS with mice overexpressing mutated APP and presenilin 1 in neurons (APP/PS1), a model of amyloid pathology, they observed an increase in phosphorylated tau (ptau) compared to APP/PS1 mice or eNOS−/− mice.…”

“…These results are surprising since NO has been reported to have the opposite effect, that is to enhance tau phosphorylation via activation of GSK3β 5 . Nevertheless, Austin et al 3 broaden our thinking about the biology of tau and raise the novel prospect that diffusible mediators arising from vascular cells protect neurons from tau phosphorylation and, potentially, pathogenicity.…”

“…Much of the attention in this field has focused on neurons and on neuronal mechanisms responsible for Aβ cleavage from the amyloid precursor protein (APP) and for tau phosphorylation, misfolding and aggregation 2 . In this issue of the journal, however, Austin and Colleagues 3 shift the focus from neurons to cerebral blood vessels and provide evidence that the endothelial isoform of nitric oxide (NO) synthase (eNOS) protects neurons from tau phosphorylation, thought to be a key step in tau pathogenicity 4 . While the findings implicate for the first time the endothelium in the molecular pathology of tau, they also provide further evidence of the importance of vascular factors in the expression of AD pathology.…”