Umami-induced obesity and metabolic syndrome is mediated by nucleotide degradation and uric acid generation

Abstract

Umami refers to the savoury taste that is mediated by monosodium glutamate (MSG) and enhanced by inosine monophosphate and other nucleotides. Umami foods have been suggested to increase the risk for obesity and metabolic syndrome but the mechanism is not understood. Here we show that MSG induces obesity, hypothalamic inflammation and central leptin resistance in male mice through the induction of AMP deaminase 2 and purine degradation. Mice lacking AMP deaminase 2 in both hepatocytes and neurons are protected from MSG-induced metabolic syndrome. This protection can be overcome by supplementation with inosine monophosphate, most probably owing to its degradation to uric acid as the effect can be blocked with allopurinol. Thus, umami foods induce obesity and metabolic syndrome by engaging the same purine nucleotide degradation pathway that is also activated by fructose and salt consumption. We suggest that the three tastes-sweet, salt and umami-developed to encourage food intake to facilitate energy storage and survival but drive obesity and diabetes in the setting of excess intake through similar mechanisms.

Extended Data Fig. 1 |. MSG is a more potent inducer of metabolic syndrome than fructose.

A) Mean daily water intake, B) daily food intake and C) total caloric intake in wild type mice on water control or exposed to the same amount of fructose or MSG (300 mM) for 15 weeks. D) Body weight gain in the same groups as in A). E) Representative H&E images of liver and F) epididymal adipose in the same groups as in A). Size bar: 20 μm G) Liver weight, H) intrahepatic triglycerides and I) plasma AST levels in the same groups as in A). J) Epididymal adipose weight and K) crown structure scoring as well as L) fasting plasma levels of insulin and M) leptin in the same groups as in A). Data in A-B and D-M represent individual points with mean ± SEM. Data in C represent mean ± SEM. One Way ANOVA. N = 6 mice per group.

Extended Data Fig. 2 |. Tamoxifen does not interfere in metabolic syndrome induced by MSG.

A) Representative western blot in liver for AMPD2 (top) and actin loading control (bottom) in tamoxifen-resistant AMPD2^Fl/FL mice and tamoxifen-inducible AMPD2 knockout mice (AMPD2^Fl/FLxCre-UBC) at days 0, 5 and 12 after tamoxifen treatment. B) Seven hour food intake in AMPD2^Fl/FL (black) and AMPD2^Fl/FLxCre-UBC (red) mice before (weeks 0 to week 7) and after (week 7 to week 15) tamoxifen (TMX) treatment C) MSG solution drinking in the same groups as in B) D) Body weight gain in in the same groups as in B) E) Fat/fat free mass ratio in in the same groups as in B) F) Fasting plasma insulin in AMPD2^Fl/FL (black) and AMPD2^Fl/FLxCre-UBC (red) mice at baseline (week 0), before TMX treatment (week 7) and after TMX treatment (week 15). G) Fasting plasma leptin levels in the same groups as in G). H) Leptin sensitivity in AMPD2^Fl/FL mice at baseline (week 0), and after TMX treatment (weeks 9 and 15). I) Leptin sensitivity in AMPD2^Fl/FLxCre-UBC mice at baseline (week 0), before TMX treatment (weeks 9 and 15). Data in F-G represent individual points with mean ± SEM. Data in B-E represent mean ± SEM. Data in H-I represent the mean. One Way ANOVA (F-G) and 2-tail t-test (A-E and H-I). N = 6 mice per group.

Extended Data Fig. 3 |. Leptin sensitivity in wild type mice exposed to MSG + IMP for 30 weeks.

A) Seven hour food intake after leptin injection in mice on water control, or exposed to IMP (300 μM), MSG (30 mM) or a MSG + IMP combination for 30 weeks and injected with leptin. B) Hypothalamic uric acid and C) TBAR levels in mice on water control, or exposed to IMP (300 μM), MSG (30 mM) or a MSG + IMP combination for 15 weeks. D) Hypothalamic mRNA levels of the leptin receptor (lepr), and cytokines il6 and tnfa. Data in B-D represent individual points with mean ± SEM. Data in A represent the meanOne Way ANOVA (B-D) and 2-tail t-test (A). N = 6 mice per group.

Extended Data Fig. 4 |. Downstream products of IMP exacerbate MSG-induced metabolic syndrome.

A) Body weight gain and B) cumulative total caloric intake in wild type mice on water control, or exposed to IMP, inosine, hypoxanthine, uric acid or allantoin (300 μM) for 30 weeks. C) Body weight gain and D) total caloric intake in wild type mice on water control, or exposed to MSG (30 mM) alone or in combination with IMP, inosine, hypoxanthine, uric acid or allantoin (300 μM) for 30 weeks. E) Epididymal adipose weight, F) liver weight, G) representative H&E images of liver, H) intrahepatic triglycerides and I) plasma AST, J) insulin and K) leptin levels in wild type mice of the same groups as in C). Size bar: 20 μm. Data in E-F, H-K represent individual points with mean ± SEM. Data in B and D represent mean ± SEM. Data in A and C represent the meanOne Way ANOVA. N = 6 mice per group.

Extended Data Fig. 5 |. Allopurinol ameliorates metabolic syndrome induced by MSG + IMP.

A) Plasma and B) intrahepatic uric acid levels in wild type mice on water control, allopurinol (AP, 1.1 mM), MSG (30 mM) alone, or in combination with IMP (300 μM) or IMP + allopurinol (AP, 1.1 mM) for 30 weeks. C) Cumulative total caloric intake and D) body weight gain in the same mice as in A). E) Epididymal adipose weight, F) liver weight, G) representative H&E images of liver, H) intrahepatic triglycerides and I) plasma AST, J) insulin and K) leptin levels in wild type mice of the same groups as in A). Size bar: 20 μm. Data in A-B and D-K represent individual points with mean ± SEM. Data in C represent mean ± SEMOne Way ANOVA (A-C, F, H-J and N-O) and 2-tail t-test (D-E, G, K-M and P-Q). N = 6 mice per group.

Extended Data Fig. 6 |. MSG and IMP activate astrocytes and microglia in the hypothalamus.

A) Representative western blot and B) densitometry for GFAP (astrocyte marker), Iba1 (microglia marker) and actin loading control in hypothalamus of wild type mice exposed to water, IMP, MSG (30 mM) or MSG + IMP. Data in B-C represent individual points with mean ± SEM. One Way ANOVA. N = 6 mice per group.

Fig. 1 |. MSG stimulates intake by promoting nucleotide turnover and purine degradation into uric acid.

a–d, Mean water intake (a), mean daily food intake (two weeks) (b), intrahepatic uric acid levels (c) and intrahepatic ATP acid levels (d) in mice exposed to increased concentrations of MSG. e, Schematic depicting the purine degradation pathway from AMP. AMP produced from the degradation of ATP is deaminized to IMP by AMPD2. IMP is then degraded to uric acid through several enzymatic reactions, producing intermediate products including inosine, hypoxanthine and xanthine. f, Intrahepatic AMPD activity in mice exposed to increased concentrations of MSG. g, Representative western blot against AMPD2 and actin control from liver extracts from WT and AMPD2 knockout mice. h, Water/MSG (300 mM) preference ratios in WT and AMPD2 knockout mice. i–l, Daily water intake (i), daily food intake (j), intrahepatic uric acid levels (k) and intrahepatic ATP levels (l) in WT and AMPD2 knockout mice exposed 300 mM of MSG. Data represent individual points with the mean ± s.e.m. One-way ANOVA analysis. n = 6 mice per group.

Fig. 2 |. Neuronal AMPD2 drives the intake of MSG and total calories.

a, Representative western blot for AMPD2 (top) and actin loading control (bottom) in WT and whole-body AMPD2 knockout mice. b, Representative western blot for AMPD2 (top) and actin loading control (bottom) in neuron-specific AMPD2 knockout mice (AMPD2^loxP/loxP × Cre-synapsin) in liver (not silenced), whole brain and hypothalamus. Neuron-specific AMPD2 knockout mice demonstrated a much greater depletion in AMPD2 expression in hypothalamic areas compared to whole brain. c, MSG/water preference ratio in AMPD2^loxP/loxP control mice and neuron-specific AMPD2 knockout mice (MSG 300 mM). d, MSG solution drinking in AMPD2^loxP/loxP control mice and neuron-specific AMPD2 knockout mice. e, Mean daily food intake in control and MSG-receiving AMPD2^loxP/loxP control mice and neuron-specific AMPD2 knockout mice. f, Hypothalamic uric acid levels in control and MSG-receiving AMPD2^loxP/loxP control mice and neuron-specific AMPD2 knockout mice. g, Hypothalamic ATP levels in control and MSG-receiving AMPD2^loxP/loxP control mice and neuron-specific AMPD2 knockout mice. Data represent individual points with the mean ± s.e.m. One-way ANOVA. n = 6 mice per group.

Fig. 3 |. Hepatocyte-specific AMPD2 drives MSG-induced metabolic syndrome and intake.

a, Representative western blot in liver and control kidney lysates for AMPD2 (top) and actin loading control (bottom) in control AMPD2^loxP/loxP, liver-specific AMPD2 knockout mice (AMPD2^loxP/loxP × Cre-albumin) and whole-body AMPD2 knockout mice. b, MSG/water preference ratio in AMPD2^loxP/loxP control mice and liver-specific AMPD2 knockout mice. c–e, MSG solution drinking in AMPD2^loxP/loxP control (200 mM) mice and liver-specific AMPD2 knockout mice (300 mM) (c), daily MSG consumption in matched AMPD2^loxP/loxP control (200 mM) mice and liver-specific AMPD2 knockout mice (d) and mean daily food intake (e) in control and MSG-receiving AMPD2^loxP/loxP mice and liver-specific AMPD2 knockout mice. f–h, Body weight gain (f), fat/fat-free mass ratio (g) and liver weight (h) in control and MSG-receiving AMPD2^loxP/loxP control mice and liver-specific AMPD2 knockout mice. i, Representative H&E liver images in control and MSG-receiving AMPD2^loxP/loxP mice and liver-specific AMPD2 knockout mice. Size bar, 20 μm. j,k, Plasma insulin (j) and leptin (k) levels in control and MSG-receiving AMPD2^loxP/loxP mice and liver-specific AMPD2 knockout mice. Data represent individual points with the mean ± s.e.m. One-way ANOVA. n = 6 mice per group.

Fig. 4 |. AMPD2 deletion ameliorates metabolic syndrome in MSG-receiving adult mice.

a, Representative western blot in the liver for AMPD2 (top) and actin (bottom) loading control in tamoxifen-inducible AMPD2 knockout mice (AMPD2^loxP/loxP × Cre-UBC) at days 0, 5 and 12 after tamoxifen treatment. b, Representative western blot in the liver, kidney hypothalamus and skeletal muscle (negative control for AMDP2 expression) for AMPD2 (top) and actin (bottom) loading control in tamoxifen-inducible AMPD2 knockout mice (AMPD2^loxP/loxP × Cre-UBC) at day 12 after receiving vehicle (A) or tamoxifen (B) treatment. c, MSG solution drinking in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. d, Daily food intake in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. e, Body weight gain in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. f, Fat/fat-free mass ratio in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. g, Representative H&E liver images in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. Size bar, 20 μm. h–j, Liver triglycerides (h), plasma insulin (i) and leptin (j) levels in AMPD2^loxP/loxP × Cre-UBC mice after vehicle (black) or tamoxifen (red) treatment. h–j, Data represent individual points with the mean ± s.e.m. c–f, Data represent the mean ± s.e.m. One-way ANOVA (h–j) or two-tailed t-test (c–f). n = 6 mice per group.

Fig. 5 |. IMP exacerbates MSG-mediated preference and food intake in WT and AMPD2 knockout mice.

a–c, Preference ratio (a), mean daily water intake (b) and mean daily food intake (c) in WT mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination. d,e, AMPD activity in the liver (d) and hypothalamus (e) of mice exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination. f, Representative western blot for AMPD2 and actin loading control in the liver and hypothalamus of mice exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination. g–i, Preference ratio (g), mean daily water intake (h) and mean daily food intake (i) in AMPD2 knockout mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination. j–l, Preference ratio (j), daily water intake (k) and daily food intake (l) in WT and AMPD2 knockout mice exposed to an MSG + IMP combination. For these studies, a two-bottle preference test was used for the studies reported in Fig. 5a,d,g. The remaining studies were done using a single-bottle preference test. Data represent individual points with the mean ± s.e.m. One-way ANOVA. n = 6 mice per group.

Fig. 6 |. MSG + IMP combination induces metabolic syndrome in mice.

a, Body weight gain of WT mice on water control or if exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 30 weeks. b–d, Epididymal adipose weight (b), representative H&E images of epididymal adipose weight (c) and adipose crown structure scoring (d) in WT mice of the same groups as in a. Size bar, 20 μm. e–i, Liver weight (e), representative H&E images of liver (f), intrahepatic triglycerides (g), plasma AST levels (h) and fasting insulin (i) in WT mice of the same groups as in a. Size bar, 20 μm. j,k, Plasma glucose (j) and area under the curve (AUC) (k) in WT mice of the same groups as in a under OGTTs. l,m, Plasma glucose (l) and AUC (m) in WT mice of the same groups as in a under ITTs. a–k,l,m, Data represent individual points with the mean ± s.e.m. j,l, Data represent the mean ± s.e.m. a–k,i,m, One-way ANOVA. n = 6 mice per group.

Fig. 7 |. MSG + IMP combination reduces leptin sensitivity in mice.

a, Total caloric intake of WT mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 30 weeks. b,c, Fasting plasma leptin (b) and epididymal adipose (c) leptin mRNA levels in WT mice of the same groups as in a. d,e, Body weight gain (d) and plasma leptin levels (e) over the two-week period in WT mice of the same groups as in a. f, Representative western blot and densitometry against total and active/phosphorylated hypothalamic STAT3 of mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 2 weeks and injected with leptin. g, Seven-hour food intake after leptin injection in mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 2 weeks and injected with leptin. h,i, Hypothalamic uric acid (h) and TBARS (i) levels in mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 2 weeks. j, Hypothalamic mRNA levels of LEPR and the cytokines IL-6 and TNF in the same mice as in g. k,l, Body weight gain (k) and plasma leptin levels (l) over a 3-d period in WT mice of the same groups as in a. m, Seven-hour food intake after leptin injection in mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 3 d and injected with leptin. n,o, Hypothalamic uric acid (n) and TBAR (o) levels in mice on water control or exposed to IMP (300 μM), MSG (30 mM) or an MSG + IMP combination for 3 d. p,q, Seven-hour food intake after leptin injection in WT, whole-body AMPD2 knockout and neuron mice or hepatocyte-specific AMPD2 knockout mice on MSG (300 mM) (p) or an MSG + IMP combination for 3 d and injected with leptin (q). b,c,f,h–j,n,o, Data represent individual points with the mean ± s.e.m. a, Data represent the mean ± s.e.m. d,e,g,k–m,p,q, Data represent the mean. a–c,f,h–j,n,o One-way ANOVA. d,e,g,k–m,p,q, Two-tailed t-test. n = 6 mice per group.

Fig. 8 |. Proposed mechanism for MSG-induced metabolic syndrome.

The addition of purines to MSG markedly exacerbates the intake of glutamate. Excessive glutamate is metabolized in the liver and hypothalamus to glutamine and ultimately IMP via the purine pathway with consumption of ATP and formation of AMP. IMP formed from both glutamine and AMP via AMPD2 enters the purine degradation pathway to generate uric acid and consequently oxidative stress and inflammation. Activation of AMPD2 in the liver drives metabolic dysfunction and adiposity while providing uric acid and IMP. Activation of AMPD2 in neurons induces inflammation and oxidative stress and reduces the sensitivity of hypothalamic neurons to leptin causing hyperphagia. Increased caloric intake and the promotion of metabolic dysfunction are the underlying factors whereby MSG and purines may cause metabolic syndrome.