Hypothalamic Paraventricular Nucleus Angiotensin II‐Mediated Microglial Activation through AT1r‐TLR4 Crosstalk in Neurogenic Hypertension

Abstract

Neurogenic hypertension is characterized by heightened sympathetic drive to the cardiovascular system from key brain regions, including the hypothalamic paraventricular nucleus (PVN). Dysregulation of Angiotensin II (AngII) during hypertension is linked to elevated synthesis of pro‐inflammatory cytokines in the PVN, suggesting a role for AngII in modulating sympathoexcitation through neuroinflammation. We have recently shown a functional interaction between AngII type‐I receptor (AT1r) and Toll‐like receptor 4 (TLR4) in mediating AngII‐dependent microglial activation and associated oxidative stress within the PVN. Our goal was to elucidate whether a similar AngII‐dependent AT1r‐TLR4 interaction contributes to the maintenance of neurogenic hypertension. Spontaneously hypertensive rats (SHRLos) were treated by oral gavage with an AT1r‐blocker (Losartan; 20mg/kg/day; 4 weeks) or vehicle (SHR), using age‐matched Wistar Kyoto rats (WKY) as a control. Tail‐cuff blood pressure measurements were taken weekly over the 4‐week treatment period. Protein expression of TLR4 and microglia‐marker IBA1 was determined by immunofluorescence (IF) within the PVN. IF skeletal analysis was used to index microglia morphology in the PVN. Real time PCR was performed for TLR4 mRNA in isolated microglia from the hypothalamus. After 4 weeks, SHR mean arterial pressure (MAP) was significantly greater than WKY controls (155±2 vs 100±3), whereas SHRLos MAP showed no significant difference from WKY (104±2 mmHg). IF analysis of PVN slices showed a 41% increase in TLR4 protein density and a 37% increase in TLR4 intensity in SHR compared with WKY, while AT1r‐blockade preserved normative TLR4 protein expression in SHRLos. Co‐localization of TLR4 with IBA1 was significantly elevated in SHR as compared to WKY (66±0.9 vs 1.3±0.2AU) and TLR4 gene expression in isolated microglia was increased 2.56‐fold in SHR compared to WKY. Microglia skeletal analysis showed a significant decrease in end‐points/frame by 21% and a 34% decrease in total branch length/frame in SHR PVN compared to WKY, indicative of increased microglial activation. SHRLos end‐point and branch‐length values were similar to WKY, demonstrating significant attenuation of microglial activation. In summary, blocking AT1r during hypertension ameliorated the elevated TLR4 protein expression seen in SHR, and normalized the extent of both microglial recruitment and activation. These findings implicate a crosstalk interaction between AngII and TLR4, via AT1r, in mediating AngII‐dependent microglial activation in hypertension. This data suggests that the characteristic sympathoexcitation of neurogenic hypertension may result, at least in part, from low‐grade chronic inflammation due to AngII‐AT1r‐TLR4 crosstalk promoting microglial activation during hypertension.Support or Funding InformationThis work was funded by AHA14SDG20400015 to VCB.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.