Institute of Biomedical Sciences

Title

A Subfornical Organ‚ÜíParaventricular Nucleus Neuronal Network Contributes to Non-Alcoholic Fatty Liver Disease via Hepatic Sympathetic Outflow

Document Type

Poster

Abstract Category

Neuroscience

Keywords

Obesity, non-alcoholic fatty liver disease, Subfornical Organ, Paraventricular nucleus of the hypothalamus, sympathetic nervous system

Publication Date

Spring 5-1-2019

Abstract

Non-alcoholic fatty liver disease (NAFLD), characterized by hepatic steatosis, leads to an increased risk for metabolic and cardiovascular diseases. Our recent work indicates that obesity-induced NAFLD is mediated by elevations in hepatic sympathetic nerve activity. However, the neural pathways that contribute to hepatic sympathetic overactivity and subsequent NAFLD development remain unknown. The subfornical organ (SFO), a circumventricular region that lies outside the blood brain barrier, senses circulating stimuli. The SFO sends dense, excitatory projections to the paraventricular nucleus of the hypothalamus (SFO→PVN), an integrative nucleus with direct hepatic spinal projections. Taken together, we hypothesized that an SFO→PVN excitatory neuronal network causes hepatic steatosis via elevations in liver sympathetic outflow. In male C57Bl/6 mice, an intersectional viral strategy was used in which a retrograde transported canine adenovirus was targeted to the PVN to express Cre-recombinase in SFO→PVN neurons (CAV2-Cre-GFP). This was combined with SFO-targeted delivery of a Cre inducible designer receptors engineered against designer drugs (DREADDs) excitatory construct. The pharmacological ligand clozapine-N-oxide (CNO; 3 mg/kg) was administered daily over 6 days to activate SFO→PVN neurons (n=4). Short-term activation of SFO→PVN neurons resulted in hepatic steatosis (2.6±0.02 vs 2.9±0.02 density*107, saline vs CNO, p<0.05) that was paralleled by elevations in liver tyrosine hydroxylase protein (1.8±0.4 CNO fold saline, p<0.05), the rate-limiting enzyme for catecholamine synthesis within postganglionic nerve terminals. Based on this, we combined the above viral strategy with selective hepatic denervation or sham surgery (n=5-6). Oil Red O staining demonstrated that hepatic denervation prevented liver lipid accumulation in response to 6-day activation of SFO→PVN neurons (1.9±0.01 vs 1.7±0.01 density*107, CNO sham vs CNO denervation, p<0.05). Importantly, this occurred independent of changes in body weight and food intake. Hepatic denervation was confirmed by a reduction in liver tyrosine hydroxylase protein expression (0.3±0.1 vs 1.8±0.3 vs 0.4±0.1-fold saline sham, saline denervation vs CNO sham vs CNO denervation, all p<0.05). Lastly, male C57Bl/6 mice that were fed a high fat diet (10 wks) underwent intersectional viral targeting for expression of an inhibitory DREADDs construct in SFO→PVN neurons. Remarkably, acute inhibition of SFO→PVN neurons in obese mice resulted in an approximate 45% reduction in hepatic steatosis (n=4/group). Collectively, these findings indicate that activation of SFO→PVN neurons causes liver triglyceride deposition via hepatic sympathetic outflow. Furthermore, in the context of obesity, inhibition of this forebrain-hypothalamic autonomic circuit results in a marked reduction in NAFLD.

Open Access

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Presented at Research Days 2019.

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A Subfornical Organ‚ÜíParaventricular Nucleus Neuronal Network Contributes to Non-Alcoholic Fatty Liver Disease via Hepatic Sympathetic Outflow

Non-alcoholic fatty liver disease (NAFLD), characterized by hepatic steatosis, leads to an increased risk for metabolic and cardiovascular diseases. Our recent work indicates that obesity-induced NAFLD is mediated by elevations in hepatic sympathetic nerve activity. However, the neural pathways that contribute to hepatic sympathetic overactivity and subsequent NAFLD development remain unknown. The subfornical organ (SFO), a circumventricular region that lies outside the blood brain barrier, senses circulating stimuli. The SFO sends dense, excitatory projections to the paraventricular nucleus of the hypothalamus (SFO→PVN), an integrative nucleus with direct hepatic spinal projections. Taken together, we hypothesized that an SFO→PVN excitatory neuronal network causes hepatic steatosis via elevations in liver sympathetic outflow. In male C57Bl/6 mice, an intersectional viral strategy was used in which a retrograde transported canine adenovirus was targeted to the PVN to express Cre-recombinase in SFO→PVN neurons (CAV2-Cre-GFP). This was combined with SFO-targeted delivery of a Cre inducible designer receptors engineered against designer drugs (DREADDs) excitatory construct. The pharmacological ligand clozapine-N-oxide (CNO; 3 mg/kg) was administered daily over 6 days to activate SFO→PVN neurons (n=4). Short-term activation of SFO→PVN neurons resulted in hepatic steatosis (2.6±0.02 vs 2.9±0.02 density*107, saline vs CNO, p<0.05) that was paralleled by elevations in liver tyrosine hydroxylase protein (1.8±0.4 CNO fold saline, p<0.05), the rate-limiting enzyme for catecholamine synthesis within postganglionic nerve terminals. Based on this, we combined the above viral strategy with selective hepatic denervation or sham surgery (n=5-6). Oil Red O staining demonstrated that hepatic denervation prevented liver lipid accumulation in response to 6-day activation of SFO→PVN neurons (1.9±0.01 vs 1.7±0.01 density*107, CNO sham vs CNO denervation, p<0.05). Importantly, this occurred independent of changes in body weight and food intake. Hepatic denervation was confirmed by a reduction in liver tyrosine hydroxylase protein expression (0.3±0.1 vs 1.8±0.3 vs 0.4±0.1-fold saline sham, saline denervation vs CNO sham vs CNO denervation, all p<0.05). Lastly, male C57Bl/6 mice that were fed a high fat diet (10 wks) underwent intersectional viral targeting for expression of an inhibitory DREADDs construct in SFO→PVN neurons. Remarkably, acute inhibition of SFO→PVN neurons in obese mice resulted in an approximate 45% reduction in hepatic steatosis (n=4/group). Collectively, these findings indicate that activation of SFO→PVN neurons causes liver triglyceride deposition via hepatic sympathetic outflow. Furthermore, in the context of obesity, inhibition of this forebrain-hypothalamic autonomic circuit results in a marked reduction in NAFLD.