Inhibition of a Forebrain‐Hypothalamic Circuit Ameliorates Hepatic Steatosis During Obesity
Authors: Blackmore Katherine , Simonyan Hayk , Young Colin N. 
Published: April 2020

Directly associated with the obesity epidemic, non‐alcoholic fatty liver disease (NAFLD) affects 1 in 3 American adults. NAFLD is characterized by hepatic triglyceride accumulation (i.e. hepatic steatosis) and leads to an increased risk for type II diabetes, insulin resistance and obesity‐related mortality. We recently demonstrated that obesity‐induced NAFLD is mediated by elevations in hepatic sympathetic nerve activity. However, the neural circuits that drive liver sympathetic overactivity remain unknown. The paraventricular nucleus of the hypothalamus (PVN) plays a critical role in autonomic regulation and has direct spinal projections to the liver. The PVN receives dense excitatory projections from the subfornical organ (SFO), a forebrain sensory circumventricular region situated outside of the blood‐brain‐barrier. Thus, we reasoned that an SFO to PVN (SFO→PVN) network is uniquely situated to mediate NAFLD, and hypothesized that inhibition of SFO→PVN neurons would reduce obesity‐induced NAFLD. Six wk old male C57Bl/6J mice were fed a normal chow or high fat diet (HFD) for 8 wks. Intersectional viral targeting was then performed in which a retrograde transported canine adenovirus was microinjected into the PVN to express Cre‐recombinase in SFO→PVN neurons (CAV2‐Cre‐GFP), combined with SFO‐targeted delivery of a Cre‐inducible designer receptors engineered against designer drugs (DREADDs) inhibitory construct (AAV2‐DIO‐hM3GimCherry). Following surgical recovery, the pharmacological ligand clozapine‐N‐oxide (CNO; 3 mg/kg i.p.) was administered once daily over 6 days to inhibit SFO→PVN neurons (n=9–13/group). Saline served as a control (n=9–10/group). One wk inhibition of SFO→PVN neurons did not influence body weight (43±2 vs. 42±2 g, HFD saline vs CNO, p>0.05), food intake, energy expenditure, or plasma free fatty acids in either normal chow or HFD animals. However, HFD resulted in significant increases in liver weight (1.3±0.1 vs. 1.9±0.1g, normal chow saline vs. HFD saline, p<0.05) and selective inhibition of SFO→PVN neurons rescued HFD‐induced hepatomegaly (1.4±0.1 g; p>0.05 vs. normal chow saline). In line with this, hepatic triglyceride quantification and histological examination (Oil Red O staining) revealed widespread hepatic lipid accumulation in HFD fed mice, which was reduced by ~70% following SFO→PVN neuronal inhibition (78.8±16 vs. 24±7 a.u. fold normal chow saline, HFD saline vs. CNO, p<0.05). Concomitant with this, inhibition of SFO→PVN neurons in obese mice was associated with an upregulation in hepatic mRNA markers of β‐oxidation (CPT1a: 1.1±0.4 vs. 4.7±0.9 fold normal chow saline, HFD saline vs CNO, p<0.05) and very‐low density lipoprotein export (APOB: 2.2±0.4 vs. 8.6±3.8 fold normal chow saline, HFD saline vs CNO, p<0.05). Collectively, these findings indicate that inhibition of SFO→PVN neurons during obesity reduces triglyceride deposition in the liver potentially by upregulating lipid disposal pathways. Furthermore, these studies suggest that manipulating this forebrain‐hypothalamic circuit, in the context of obesity, may be a novel approach to target NAFLD.

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1R01DK117007, 1R01HL141393