Abstract 025: Cellular Mechanisms of Blood-Brain Barrier Disruption in Ang II-Induced Hypertension

The blood-brain barrier (BBB) is critically important for brain health by regulating molecular exchanges between blood and brain. Hypertension (HTN) induces breakdown of the BBB which may contribute to its deleterious effect on the brain, but the cellular bases of the BBB opening remain to be established. We used a model of angiotensin II (AngII) HTN (600ng/kg/min x 2 weeks; n>5/group) to investigate the mechanisms of the BBB opening. BBB permeability was assessed spectrophotometrically in C57BL/6J mice using 3000 MW FITC-dextran as a tracer. HTN increased BBB permeability in Ang II HTN (29.8 ± 0.9 vs 18.1 ± 0.5 ng/g in control, p<0.01), an effect partially ameliorated by the angiotensin type 1 receptor (AT1R) antagonist losartan in the drinking water (22.9 ± 0.6 ng/g) but not by hydralazine + hydrochlorothiazide (27.4 ± 0.7 ng/g), suggesting involvement of AT1R and not elevated blood pressure. Next, we sought to identify the mechanisms of the breakdown of the BBB in HTN. First, we used electron microscopy to examine the ultrastructure of endothelial tight junctions (TJ) and assess vesicular transport, key components of the BBB. HTN reduced the length (-25%) and complexity (-11%) of TJ, and increased the number of endothelial vesicles (2.22 vs 1.42 vesicles/endothelial area, p<0.05). The TJ remodeling was associated with a reduction in occludin (-17%, p<0.01) and claudin-5 (-30%, p<0.05) mRNA in microvascular preparations. Additionally, the expression of Mfsd2a, a lipid transporter that also suppresses vesicular transcytosis, was markedly attenuated (-43%, p<0.01). Taken together, these data suggest a strong effect of Ang II on cerebral endothelial cells to induce BBB opening. Since perivascular macrophages (PVM) mediate cerebral endothelial dysfunction in HTN (J Clin Invest 2016;126:4674), we tested their involvement in the BBB opening. PVM depletion with icv clodronate or deletion of AT1 receptors in PVM partially attenuated the BBB opening in Ang II HTN (p<0.05). Thus, we conclude that AT1R in cerebral endothelial cells and PVM mediate the BBB opening by targeting both paracellular and vesicular transport. Such increase in BBB permeability to circulating agents may contribute to the cerebrovascular and cognitive dysfunction associated with HTN.