Abstract 23: Cognitive impairment during chronic i.c.v. angiotensin II is associated with increased tau phosphorylation in the cortex and hippocampus

Abstract

Cardiovascular risk factors, such as hypertension, are recognized as important contributors to cognitive impairment and dementia, including Alzheimer’s disease (AD). However, the mechanisms underlying the detrimental effects of hypertension on the brain remain to be fully established. Angiotensin II (Ang II) has known detrimental effects on the brain, including cerebrovascular damage, sympathetic activation, and neuroinflammation. Ang II activates neuronal glycogen synthase 3b (GSK-3b), a major kinase involved in the phosphorylation of the microtubule associated protein tau. Hyperphosphorylated tau and the subsequent formation of neurofibrillary tangles are a hallmark of AD and related dementias (tauopathies). Here, we tested the central hypothesis that brain Ang II leads to cognitive impairment by increasing tau hyperphosphorylation. 3-month old male C57BL6 mice were implanted with a s.c. osmotic minipump connected to an i.c.v. cannula delivering low dose Ang II (6ng/hr) or saline for 4 weeks. Blood pressure was monitored twice weekly using tail-cuff plethysmography. During the 4 th week of infusion, mice underwent a battery of neurobehavioral testing assessing various cognitive domains including novel object recognition (recognition and short-term memory), Y-maze (spatial working memory), Barnes maze (spatial learning and long-term memory), and hippocampal/executive function (nest building). Tau phosphorylation was assessed in the cortex and hippocampus by Western blotting. Systolic blood pressure increased slightly by week 3 prior to start of behavioral tests (control: 115.3 ± 4.1 mmHg vs Ang II: 130.8 ± 4.7; n=10). We did not observe significant differences during novel object recognition (discrimination index: control: 0.49 ± 0.08 vs Ang II: 0.35 ± 0.23; n=5) or Y-maze (spontaneous alternations: control: 62.4 ± 6.1% vs Ang II: 59.0 ± 4.3%). Mice receiving i.c.v. Ang II had significantly delayed latency to find the escape hole during the Barnes maze learning trials (Treatment p=0.0006, Time p=0.0039, interaction: p=0.3028; two-way repeated measures ANOVA). The impairment in spatial learning was associated with an increase in tau phosphorylation in the cortex assessed by AT8 antibody. Ongoing studies are investigating whether this increase is mediated by activation of neuronal GSK-3b. We conclude that Ang II-mediated hyperphosphorylation of tau is a potential novel molecular mechanism driving the detrimental effects of hypertension on cognitive health.