Expression of melanin-concentrating hormone receptor 2 protects against diet-induced obesity in male mice

Abstract

Melanin-concentrating hormone (MCH) is an orexigenic neuropeptide that is a ligand for two subtypes of MCH receptors, MCHR1 and MCHR2. MCHR1 is universally expressed in mammals ranging from rodents to humans, but the expression of MCHR2 is substantially restricted. In mammals, MCHR2 has been defined in primates as well as other species such as cats and dogs but is not seen in rodents. Although the role of MCHR1 in mediating the actions of MCH on energy balance is clearly defined using mouse models, the role of MCHR2 is harder to characterize because of its limited expression. To determine any potential role of MCHR2 in energy balance, we generated a transgenic MCHR1R2 mouse model, where human MCHR2 is coexpressed in MCHR1-expressing neurons. As shown previously, control wild-type mice expressing only native MCHR1 developed diet-induced obesity when fed a high-fat diet. In contrast, MCHR1R2 mice had lower food intake, leading to their resistance to diet-induced obesity. Furthermore, we showed that MCH action is altered in MCHR1R2 mice. MCH treatment in wild-type mice inhibited the activation of the immediate-early gene c-fos, and coexpression of MCHR2 reduced the inhibitory actions of MCHR1 on this pathway. In conclusion, we developed an experimental animal model that can provide insight into the action of MCHR2 in the central nervous system and suggest that some actions of MCHR2 oppose the endogenous actions of MCHR1.

Figure 1.

Expression of hMCHR2 in the brains of MCHR1R2 mice. A, Schematic of the BAC construct used to generate the MCHR1R2 mouse. Downstream of the mMchr1 promoter, a 37-bp region of exon (ex) 1 from the mMCHR1 coding sequence was deleted (cross-hatched area) and replaced with the full-length coding sequence for human hMCHR2. B, Gene expression of mMchr1 and hMCHR2 in the brain and peripheral tissues. Gene expression in brain tissue was represented by whole hypothalamus. C, Gene expression of mMchr1 and hMCHR2 throughout the WT and MCHR1R2 brain. Expression of hMCHR2 was not detected in WT tissue and thus was omitted (B and C). **, P < .01; ***, P < .001. Abbreviations: cbell, cerebellum; ctx, cerebral cortex; eWAT, epididymal white adipose tissue; hcmp, hippocampus; hind, hindbrain; hthal, hypothalamus; mid, midbrain; musc, gastrocnemius skeletal muscle; olf, olfactory bulb; pancr, pancreas; str, striatum; thal, thalamus.

Figure 2.

MCHR1R2 mice are lean and resistant to DIO on an HFD. A and B, Body weight curve of WT and MCHR1R2 littermates on CD and HFD. C and D, Comparison of fat and lean mass over time during HFD feeding. *, P < .05; **, P < .01.

Figure 3.

MCHR1R2 mice on HFD display an improved serum metabolic profile. A, Blood glucose levels in the fed and fasted state. B, Fed serum insulin. C and D, Comparison of glucose tolerance (2 g/kg glucose, ip) and insulin tolerance (1 U/kg insulin, ip) curves after 8 weeks of HFD feeding. E–G, Fed serum free fatty acid, cholesterol, and triglyceride. *, P < .05; **, P < .01.

Figure 4.

MCHR1R2 mice are hypophagic with unaltered energy expenditure and locomotor activity. A, Cumulative food intake per mouse measured weekly. B, Feed efficiency represented as body weight gained (grams) per HFD consumed (kilocalories). C and D, Oxygen consumption (VO₂) per mouse normalized to lean mass. E, Calculated heat production. F, Locomotor activity measured by total number of ambulations along the x- and y-axis. ***, P < .001.

Figure 5.

MCHR2 expression attenuated the MCH-mediated decrease in c-fos induction at the arcuate nucleus. A, Representative photomicrographs of c-fos–immunoreactive cells in the arcuate nucleus. Scale bar, 100 μm. B, Number of c-fos–positive cells in WT and MCHR1R2 mice treated with ACSF and MCH (3 μg, icv). *, P < .05.