Activation of a Forebrain‐Hypothalamic Circuit Disrupts Hepatic Lipid Regulatory Pathways
Authors: Houchen Claire J. , Blackmore Katherine , Simonyan Hayk , Young Colin N. 
Published: June 2021

Non‐alcoholic fatty liver disease (NAFLD), characterized by an accumulation of hepatic triglycerides, is a significant contributor to chronic metabolic disorders. We have recently demonstrated a role for the central nervous system in NAFLD. Specifically, activation of a forebrain‐hypothalamic circuit that involves excitatory projections from the subfornical organ (SFO) to the paraventricular nucleus of the hypothalamus (PVN) results in elevations in liver triglycerides. However, the underlying liver metabolic pathways through which this neural circuit contributes to NAFLD remains unknown. We hypothesized that activation of excitatory SFO PVN‐projecting neurons contributes to hepatic steatosis via alterations in hepatic lipid acquisition and disposal pathways. To specifically manipulate SFO PVN‐projecting neurons, we utilized a combinatorial viral approach in male C57Bl/6 mice. A retrograde transported adenovirus was targeted to the PVN to express Cre‐recombinase in SFO PVN‐projecting neurons (CAV2‐Cre‐GFP). This was combined with SFO microinjection of a Cre‐inducible designer receptors engineered against designer drugs (DREADDs) viral construct (AAV2‐DIO‐hM3Gq‐mCherry). Following surgical recovery, the pharmacological ligand clozapine‐N‐oxide (CNO; 3 mg/kg i.p.) was administered once daily over 6 days to activate SFO PVN‐projecting neurons (n=4). Saline served as a control (n=4). Hepatic lipid accumulation is determined by a complex interplay between de novo lipogenesis, hepatic β‐oxidation, free fatty acid uptake, and triglyceride disposal in the form of very low‐density lipoprotein. Western blot analysis revealed that acyl‐coenzyme A oxidase 1 (ACOX1), an indicator of peroxisomal β‐oxidation, was not altered following activation of SFO PVN‐projecting neurons (0.8±0.03 CNO fold saline; p>0.05). In contrast, acute activation of this forebrain‐hypothalamic network resulted in elevations in hepatic carnitine palmitoyltransferase 1a (CPT1a), which is involved in mitochondrial β‐oxidation (1.4±0.1 CNO fold saline, p<0.05). In addition, liver free fatty acid transport protein cluster of differentiation 36 (2.3±0.6 CNO fold saline, p<0.05) and fatty acid synthase (1.6±0.1 CNO fold saline, p<0.05) were upregulated following activation of SFO‐PVN neurons, indicative of increased free fatty acid uptake into the liver and enhanced de novo lipogenesis, respectively. Real time qPCR analysis similarly indicated a marked upregulation of liver markers of de novo lipogenesis, gluconeogenesis, as well as very low‐density liporotein export following activation of SFO neurons that project to the PVN (e.g. DGAT1: 3.6±0.3 CNO fold saline; ApoB: 3.8±1.8 CNO fold saline, both p<0.05). Importantly, these changes occurred independent of group differences in body weight (25±1 vs. 25+1 g, saline vs. CNO, p>0.05) and food intake. Collectively, these findings indicate that activation of excitatory SFO PVN‐projecting neurons may contribute to hepatic steatosis through alterations in liver lipid metabolic pathways involved in mitochondrial β‐oxidation, de novo lipogenesis, free fatty acid uptake, and very low‐density lipoprotein export.Support or Funding InformationR01DK117007This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.