Predisposition to late-onset obesity in GIRK4 knockout mice

Abstract

G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels mediate the inhibitory effects of many neurotransmitters on excitable cells. Four Girk genes have been identified (Girk1-4). Whereas GIRK4 is associated with the cardiac GIRK channel, Girk4 expression has been detected in a few neuron populations. Here, we used a transgenic mouse expressing enhanced green fluorescent protein (EGFP) under the control of the Girk4 gene promoter to clarify the expression pattern of Girk4 in the brain. Although small subsets of EGFP-positive neurons were evident in some areas, prominent labeling was seen in the hypothalamus. EGFP expression was most pronounced in the ventromedial, paraventricular, and arcuate nuclei, neuron populations implicated in energy homeostasis. Consistent with a contribution of GIRK4-containing channels to energy balance, Girk4 knockout -/- mice were predisposed to late-onset obesity. By 9 months, Girk4-/- mice were approximately 25% heavier than wild-type controls, a difference attributed to greater body fat. Before the development of overweight, Girk4-/- mice exhibited a tendency toward greater food intake and an increased propensity to work for food in an operant task. Girk4-/- mice also exhibited reduced net energy expenditure, despite displaying elevated resting heart rates and core body temperatures. These data implicate GIRK4-containing channels in signaling crucial to energy homeostasis and body weight.

Fig. 1.

EGFP expression in the Tg(Kcnj5-EGFP)49Gsat mouse. (A and B) EGFP signal was observed in sections (15 μm) from transgene-positive (Tg+) (A) but not transgene-negative (Tg−) (B) mice. (Scale bars: 1 mm.) (C and D) Sections containing the substantia nigra pars reticulata (C) (SNr) and hippocampus (D). The neurons found in the dentate gyrus may be MOPP cells described by Somogyi and colleagues (33). h, hilus; ml, molecular layer; gc, granule cell layer. (Scale bars: C, 50μm; D, 300μm.)

Fig. 2.

EGFP expression in the hypothalamus of Tg(Kcnj5-EGFP)49Gsat mice. A series (rostral-to-caudal) of coronal sections (15 μm) through the hypothalamus from an adult male Tg(Kcnj5-EGFP)49sat mouse. Images are representative of those taken from three different Tg(Kcnj5-EGFP)49sat mice. Abbreviations are defined in Table 2. (Scale bar: 600 μm.)

Fig. 3.

Body weights of wild-type and Girk4^−/− mice. (A) Body weights of wild-type and Girk4^−/− mice at 3–10 weeks of age. Mice were housed with same-sex siblings (two to four per cage) of the same genotype. Groups ranged from 12 to 28 per genotype and gender. (B and C) Body weights of wild-type and Girk4^−/− mice measured at 3, 6, 9, and 12 months of age. The number in each bar denotes the coefficient of variation (CV%). (D) Chemical (Soxhlet) analysis of carcass composition of 12-month-old male wild-type (n = 5) and Girk4^−/− (n = 6) mice. * and **, P < 0.05 and 0.01, respectively, vs. wild-type, same gender.

Fig. 4.

Weight gain and food intake of single-housed wild-type and Girk4^−/− mice. (A and B) Body weights of single-housed male (A) and female (B) wild-type and Girk4^−/− mice between 12 and 36 weeks of age. (C and D) Daily food intake determined during 4-week intervals, including the acclimation period (weeks 9–12), for male (C) and female (D) wild-type and Girk4^−/− mice. (Insets) Cumulative food intake between 13 and 36 weeks of age. Groups ranged from six to eight per genotype and gender. *, P < 0.05 vs. wild-type, same gender.

Fig. 5.

Body temperature and heart rate in wild-type and Girk4^−/− mice. (A) Core body temperature in male wild-type and Girk4^−/− mice over a 6-h interval at 3, 6, and 9 months of age. (B) Heart rate [beat per minute (bpm)] measured over the same 6-h interval. Groups ranged from 8 to 11 per genotype and time point. * and ***, P < 0.05 and 0.001, respectively, vs. wild-type.