Epigenetic dysregulation of the dopamine system in diet-induced obesity

Abstract

Chronic intake of high-fat (HF) diet is known to alter brain neurotransmitter systems that participate in the central regulation of food intake. Dopamine (DA) system changes in response to HF diet have been observed in the hypothalamus, important in the homeostatic control of food intake, as well as within the central reward circuitry [ventral tegmental area (VTA), nucleus accumbens (NAc), and pre-frontal cortex (PFC)], critical for coding the rewarding properties of palatable food and important in hedonically driven feeding behavior. Using a mouse model of diet-induced obesity (DIO), significant alterations in the expression of DA-related genes were documented in adult animals, and the general pattern of gene expression changes was opposite within the hypothalamus versus the reward circuitry (increased vs. decreased, respectively). Differential DNA methylation was identified within the promoter regions of tyrosine hydroxylase (TH) and dopamine transporter (DAT), and the pattern of this response was consistent with the pattern of gene expression. Behaviors consistent with increased hypothalamic DA and decreased reward circuitry DA were observed. These data identify differential DNA methylation as an epigenetic mechanism linking the chronic intake of HF diet with altered DA-related gene expression, and this response varies by brain region and DNA sequence.

Figure 1. Long-term exposure to high-fat (HF) diet alters TH and DAT mRNA expression

Gene expression was measured in the hypothalamus (HYP; left) and ventral tegmental area (VTA; right) of control (white bars) and diet-induced obese (DIO) mice (black bars) using quantitative real time PCR. TH and DAT mRNA were significantly increased in the HYP, while TH and DAT mRNA were significantly decreased in the VTA. *p<.025, **p<.005 (n=7-9/group)

Figure 2. Long-term exposure to high-fat (HF) diet affects dopamine-related gene expression in the reward circuitry

Dopamine-related gene expression (D1, D2, DARPP-32, COMT) was measured in the hypothalamus (HYP) and regions of the central reward circuitry (VTA, NAc and PFC) of control and diet-induced obese (DIO) mice using quantitative real time PCR. These genes were unchanged in the HYP, but D1, D2 and DARPP-32 levels were reduced in the reward-related regions in DIO animals and the effect varied by region. COMT levels were not altered in any region studied. *p<.025, **p<.005 (n=7/group in HYP, PFC, NAc; n=4/group in VTA)

Figure 3. DNA methylation status of TH and DAT promoters in DIO mice

Genomic DNA was isolated from dissected HYP and VTA of control (white bars) and obese (DIO) mice (black bars), sheared by sonication and immunoprecipitated with 5-methylcytosine antibody. The enrichment of DNA methylation relative to input genomic DNA in the promoter region of TH or DAT was quantified by qPCR. DIO mice displayed decreased methylation of TH and DAT within the HYP. The opposite effect was observed in the VTA, with both promoters showing significantly more methylation. GAPDH methylation was not altered in DIO mice (data not shown). Values are mean ± s.e.m. *p <0.05, **p< 0.01, n=6/group, Two-tailed t-test.

Figure 4. Food intake analysis of DIO mice

(a) Food intake, (b) meal number, and (c) meal size were evaluated in control (white bars) and DIO mice (black bars) (n=4/group). DIO mice were hyperphagic, with a significant increase in food intake, meal number and meal size during the dark period (LEFT). Food intake and meal number were significantly elevated during the light period as well (RIGHT).

Figure 5. DIO mice show decreased intake of a rewarding food

Food intake was measured two separate times during the dark period in control and DIO mice, once when fed the control diet and once when fed the HF diet (counterbalanced order). Control mice increased their food intake when presented with the palatable HF diet, while DIO mice failed to show this response.