Cross talk between AT₁receptors and Toll-like receptor 4 in microglia contributes to angiotensin II-derived ROS production in the hypothalamic paraventricular nucleus

Abstract

II is thought to increase sympathetic outflow by increasing oxidative stress and promoting local inflammation in the paraventricular nucleus (PVN) of the hypothalamus. However, the relative contributions of inflammation and oxidative stress to sympathetic drive remain poorly understood, and the underlying cellular and molecular targets have yet to be examined. ANG II has been shown to enhance Toll-like receptor (TLR)4-mediated signaling on microglia. Thus, in the present study, we aimed to determine whether ANG II-mediated activation of microglial TLR4 signaling is a key molecular target initiating local oxidative stress in the PVN. We found TLR4 and ANG II type 1 (AT 1) receptor mRNA expression in hypothalamic microglia, providing molecular evidence for the potential interaction between these two receptors. In hypothalamic slices, ANG II induced microglial activation within the PVN (ϳ65% increase, P Ͻ 0.001), an effect that was blunted in the absence of functional TLR4. ANG II increased ROS production, as indicated by dihydroethidium fluorescence, within the PVN of rats and mice (P Ͻ 0.0001 in both cases), effects that were also dependent on the presence of functional TLR4. The microglial inhibitor minocycline attenuated ANG II-mediated ROS production, yet ANG II effects persisted in PVN single-minded 1-AT 1a knockout mice, supporting the contribution of a non-neuronal source (likely microglia) to ANG II-driven ROS production in the PVN. Taken together, these results support functional interactions between AT 1 receptors and TLR4 in mediating ANG II-dependent microglial activation and oxidative stress within the PVN. More broadly, our results support a functional interaction between the central renin-angiotensin system and innate immunity in the regulation of neurohumoral outflows from the PVN. ANGIOTENSIN II (ANG II) plays a critical role in fluid balance and hemodynamic regulation (25). Within the central nervous system (CNS), ANG II acting through ANG II type 1 a (AT 1a ) receptors (AT 1a R) has been shown to be implicated in the regulation of sympathetic and neuroendocrine outputs from the brain (45, 49, 50). The paraventricular nucleus (PVN) of the hypothalamus has been recognized as a critical neuronal substrate mediating central ANG II actions, thus playing a critical role in ANG II-mediated neurohumoral responses (23, 78). Indeed, a large body of evidence supports enhanced angiotensinergic actions within the CNS, including the PVN, as a key pathophysiological mechanism involved in cardiovascular diseases, including hypertension and heart failure (37,60,78,80,88). However, despite its importance, the precise mechanisms and cellular/ molecular targets underlying ANG II signaling within the CNS are incompletely understood.Inflammation and oxidative stress have emerged as novel mechanisms by which central ANG II mediates its deleterious effects. For example, studies have shown that pressor and sympathetic responses to central ANG II are mediated by superoxide within the PVN (8, 22). Likewise, several studie...

Clinical Relevance and Role of Neuronal AT 1 Receptors in ADAM17-Mediated ACE2 Shedding in Neurogenic Hypertension

“…11 As the main mediator of brain RAS over-activity, the AT 1a R was previously demonstrated to prompt ADAM17 up-regulation both in vitro 18 and in vivo. 5 Moreover, in recent years cell-specific expression of AT 1a R, in the CNS, has been shown to contribute to neurogenic hypertension 10,27,28 and other ailments. 29 Since AT 1a R and ADAM17 co-exist on various cell types, including endothelial cells, 30 vascular smooth muscle cells, 31 glial cells and neurons, 32 the contribution of cell-specific ADAM17 activation to the development of hypertension must be determined to insure selective targeting.…”

“…However, AT 1 R are expressed on various cell types in the brain, including neurons, microglia and astrocytes, which have been shown to contribute to neurogenic hypertension. [8][9][10] To further investigate the relationship between brain RAS and ADAM17 activation, we used the deoxycorticosterone acetate (DOCA)-salt model, which promotes RAS activation specifically in the brain, to test the hypothesis that neuronal AT 1a R are essential for ADAM17-mediated ACE2 shedding in neurogenic hypertension. 5,11 In patients, we observed an increase in sACE2, indicating increased ACE2 ectodomain shedding in hypertensive individuals.…”

Inhibition of microglial activation in rats attenuates paraventricular nucleus inflammation in Gαi2 protein‐dependent, salt‐sensitive hypertension

“…Several studies, conducted in different models of hypertension, including the spontaneously hypertensive rat and the angiotensin II infusion model in Sprague–Dawley rats, have reported the presence of neuroinflammation, including increased production of pro‐inflammatory cytokines (Bardgett et al., ; Biancardi et al., ; Shi et al., ). A recent study conducted in the DSS rat model also reported an upregulation of PVN pro‐inflammatory cytokines in response to high dietary sodium intake (Jiang et al., ).…”

“…Inflammatory processes have also been demonstrated to mediate neuronal excitability and even influence the firing rates of neurons, potentially disrupting neural homeostasis (Galic, Riazi, & Pittman, 2012;Shi et al, 2011). Glial cells, including microglia and astrocytes, can become activated in response to a pathological stimulus and have been demonstrated to produce inflammatory cytokines and reactive oxygen species and to disrupt neural homeostasis (Biancardi, Stranahan, Krause, de Kloet, & Stern, 2016;Wolf, Boddeke, & Kettenmann, 2017). Therefore, we hypothesized that central inflammatory processes, involving activation of microglia and astrocytes, contribute to the development of G i 2 protein-dependent, salt-sensitive hypertension.…”

Differential effect of angiotensin II and blood pressure on hippocampal inflammation in mice

“…A recent study demonstrated that isolated hypothalamic microglia actually respond to Ang II through AT1 and TLR4 receptors, the latter which is required for the expression of oxidative damage induced by Ang II in mice [49]. Although the role of other cells including neurons and astrocytes cannot be discarded, the ultrastructural morphological changes along with the increased hippocampal TNF-α production suggest the emergence of a pro-inflammatory microglial phenotype in mice with chronic Ang II administration.…”