Impaired mTORC2 signaling in catecholaminergic neurons exaggerates high fat diet-induced hyperphagia

Abstract

Objective Food intake is highly regulated by central homeostatic and hedonic mechanisms in response to peripheral and environmental cues. Neutral energy balance stems from proper integration of homeostatic signals with those “sensing” the rewarding properties of food. Impairments in brain insulin signaling causes dysregulation of feeding behaviors and, as a consequence, hyperphagia. Here, we sought to determine how the mammalian target of rapamycin complex 2 (mTORC2), a complex involved in insulin signaling, influences high fat feeding. Methods Rictor is a subunit of mTORC2, and its genetic deletion impairs mTORC2 activity. We used Cre-LoxP technology to delete Rictor in tyrosine hydroxylase (TH) expressing neurons (TH Rictor KO). We assessed food intake, body weight, body composition and DA dependent behaviors. Results TH Rictor KO mice display a high-fat diet specific hyperphagia, yet, when on low-fat diet, their food intake is indistinguishable from controls. Consistently, TH Rictor KO become obese only while consuming high-fat diet. This is paralleled by reduced brain DA content, and disruption of DA dependent behaviors including increased novelty-induced hyperactivity and exaggerated response to the psychostimulant amphetamine (AMPH). Conclusions Our data support a model in which mTORC2 signaling within catecholaminergic neurons constrains consumption of a high-fat diet, while disruption causes high-fat diet-specific exaggerated hyperphagia. In parallel, impaired mTORC2 signaling leads to aberrant striatal DA neurotransmission, which has been associated with obesity in human and animal models, as well as with escalating substance abuse. These data suggest that defects localized to the catecholaminergic pathways are capable of overriding homeostatic circuits, leading to obesity, metabolic impairment, and aberrant DA-dependent behaviors.

Deletion of Rictor in catecholaminergic neurons alters locomotor activity and ingestive behavior

“…This difference in activity may be partially explained by the fact that the total activity of female mice, regardless of genotype, was greater than that of males (Two-way ANOVA, Sex factor F(1,81)=17.9, p<0.0001). The increase in baseline locomotor activity in male homozygous Rictor-TH KO mice is consistent with published data (Dadalko et al, 2015a), however female mice were not examined in the previous study.…”

“…Experiments utilized adult male and female mice (8–15 weeks). Homozygous floxed Rictor mice were generated as previously described (Mazei-Robison et al, 2011; Shiota et al, 2006; Siuta et al, 2010), and were also crossed with heterozygous tyrosine hydroxylase (TH)-Cre mice (Jackson Laboratories, 008601) to generate developmental Rictor knock-out (KO) mice (Dadalko et al, 2015a); all mice were fully backcrossed to the c57Bl/6 background. Mouse genotypes were verified at 21–28 days using standard procedures.…”

“…Given the lack of a selective pharmacological inhibitor, we altered VTA TORC2 signaling via genetic deletion and viral-mediated overexpression of rapamycin-insensitive companion of TOR (Rictor), as this protein is a necessary component for TORC2 kinase activity. Global deletion of TORC2 is embryonically lethal (Shiota et al, 2006), and thus floxed-Rictor mice have been developed and used in combination with Cre driver lines (Dadalko et al, 2015a; Dadalko et al, 2015b; Siuta et al, 2010; Thomanetz et al, 2013) or stereotaxic infusion of AAV-Cre in adult mice (Mazei-Robison et al, 2011) to produce cell-type or brain-region specific Rictor KO mice to allow examination of the role of TORC2 signaling in vivo . Recently, floxed-Rictor mice have been crossed to the tyrosine hydroxylase (TH)-Cre reporter line to KO TORC2 signaling specifically from catecholaminergic neurons (TH-Rictor) (Dadalko et al, 2015a).…”

“…Global deletion of TORC2 is embryonically lethal (Shiota et al, 2006), and thus floxed-Rictor mice have been developed and used in combination with Cre driver lines (Dadalko et al, 2015a; Dadalko et al, 2015b; Siuta et al, 2010; Thomanetz et al, 2013) or stereotaxic infusion of AAV-Cre in adult mice (Mazei-Robison et al, 2011) to produce cell-type or brain-region specific Rictor KO mice to allow examination of the role of TORC2 signaling in vivo . Recently, floxed-Rictor mice have been crossed to the tyrosine hydroxylase (TH)-Cre reporter line to KO TORC2 signaling specifically from catecholaminergic neurons (TH-Rictor) (Dadalko et al, 2015a). TH-Rictor KO mice display an increase in novelty-induced locomotion compared to their wild-type controls, as well as an increase in lean and overall body mass, but with no reported difference in fat mass (Dadalko et al, 2015a).…”

“…Recently, floxed-Rictor mice have been crossed to the tyrosine hydroxylase (TH)-Cre reporter line to KO TORC2 signaling specifically from catecholaminergic neurons (TH-Rictor) (Dadalko et al, 2015a). TH-Rictor KO mice display an increase in novelty-induced locomotion compared to their wild-type controls, as well as an increase in lean and overall body mass, but with no reported difference in fat mass (Dadalko et al, 2015a). …”