Abstract 202: Involvement of neuronal high mobility group box 1 (HMGB1) in Angiotensin II-mediated neuronal-microglial interaction

Abstract

We have demonstrated that activation of microglia in the autonomic brain regions, primarily in the paraventricular nucleus (PVN), is critical in the development and establishment of hypertension. Neuroimmune synaptic communication is implicated in activating microglia and inducing neuroinflammation in many neuropathophysiological diseases. Specifically, HMGB1 secreted by neurons is a critical mediator in neuroimmune crosstalk. Thus we hypothesized that HMGB1 is a key regulator involved in microglia activation in Ang II-induced hypertension. Neuronal and microglial cells in primary culture were established for this study. Effect of Ang II on microglial movement was recorded and quantified by time-lapse microscopy and video analysis. In addition, the expression, translocation and secretion of HMGB1 in neurons were measured by qPCR, Western blot and immunocytochemistry. Ang II treatment of neuronal-microglial co-culture resulted in a 5-fold increase of microglial migration towards neurons, which was completely abolished by treatment with 1 μM Losartan, an Ang II receptor type 1 antagonist. The presence of neurons was found to be essential for microglial movement since Ang II failed to induce these effects in the absence of neurons. Neurons express high levels of HMGB1, which was predominantly localized in the nucleus. Ang II treatment resulted in a time-dependent translocation of nuclear hMGB1 into the cytoplasm and ultimately into the medium. Both translocation and release of Ang II-induced HMGB1 was blocked by Losartan. Furthermore treatment of microglia with 500ng/ml recombinant HMGB1 caused ~2 fold increase of proinflammatory cytokines (TNF-α and IL-1β). The importance of HMGB1 is further underscored by our observation that its protein level was increased ~2 fold in the PVN but not in the nucleus of the solitary tract (NTS) or rostral ventrolateral medulla (RVLM) of the spontaneously hypertensive rats (SHR) compared with the normotensive WKY rats. Taken together, these observations suggest that Ang II primarily activates the neuronal AT1R to increase the expression, cytosolic translocation and release of HMGB1. The released HMGB1 primes microglial activation and release of cytokines, two key events in neuroinflammation in hypertension.