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. 2013 Sep 3;18(3):445-55.
doi: 10.1016/j.cmet.2013.08.006.

K(ATP)-channel-dependent Regulation of Catecholaminergic Neurons Controls BAT Sympathetic Nerve Activity and Energy Homeostasis

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Free PMC article

K(ATP)-channel-dependent Regulation of Catecholaminergic Neurons Controls BAT Sympathetic Nerve Activity and Energy Homeostasis

Sulay Tovar et al. Cell Metab. .
Free PMC article

Abstract

Brown adipose tissue (BAT) is a critical regulator of glucose, lipid, and energy homeostasis, and its activity is tightly controlled by the sympathetic nervous system. However, the mechanisms underlying CNS-dependent control of BAT sympathetic nerve activity (SNA) are only partly understood. Here, we demonstrate that catecholaminergic neurons in the locus coeruleus (LC) adapt their firing frequency to extracellular glucose concentrations in a K(ATP)-channel-dependent manner. Inhibiting K(ATP)-channel-dependent control of neuronal activity via the expression of a variant K(ATP) channel in tyrosine-hydroxylase-expressing neurons and in neurons of the LC enhances diet-induced obesity in mice. Obesity results from decreased energy expenditure, lower steady-state BAT SNA, and an attenuated ability of centrally applied glucose to activate BAT SNA. This impairs the thermogenic transcriptional program of BAT. Collectively, our data reveal a role of K(ATP)-channel-dependent neuronal excitability in catecholaminergic neurons in maintaining thermogenic BAT sympathetic tone and energy homeostasis.

Figures

Figure 1
Figure 1. Kir6.2TH-Cre Mice Develop Diet-Induced Obesity
(A) Visualization of Cre activity in TH-IRES-Cre-LacZ reporter mice. Double immunofluorescence for endogenous TH and transgenically expressed β-galactosidase (β-gal) in different brain regions of double-heterozygous TH-IRES-Cre-LacZ reporter mice is also shown. Nuclear staining, blue (DAPI); β-gal staining, green; tyrosine hydroxylase, red (TH). Ventral tegmental area, VTA; locus coeruleus, LC; rostroventrolateral medulla, RVLM. Details are shown at a higher magnification. (B) Expression of Kir6.2 in catecholaminergic neurons. Immunofluorescence detection of Kir6.2 expression and GFP in adjacent sections of double heterozygous TH-IRES-Cre-GFP reporter mice is also shown. TH-IRES-Cre-negative littermates were used as controls. Green, Kir6.2; red, GFP. Details are shown at a higher magnification. (C) Average body weight of male control (n = 14) and Kir6.2TH-Cre mice (n = 18) on normal chow diet (ND). (D) Average body weight of male control (n = 12) and Kir6.2TH-Cre mice (n = 12) on high-fat diet (HFD). (E) Average body fat content of 20-week-old male control (n = 13) and Kir6.2TH-Cre mice (n = 14) on HFD measured by nuclear magnetic resonance. (F) Epididymal fat pad weight of 20-week-old male control and Kir6.2TH-Cre mice on ND (control, n = 12; Kir6.2TH-Cre, n = 15) and on HFD (control, n = 13; Kir6.2TH-Cre, n = 14). (G) Representative H&E staining of epididymal adipose tissue of a 20-week-old male control (upper panel) and Kir6.2TH-Cre mice (lower panel) on HFD (10× magnification in the small square). (H) Quantification of mean adipocyte surface in epididymal adipose tissue of 20-week-old male control (n = 4) and Kir6.2TH-Cre mice (n = 4) on HFD. Data are expressed as mean ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001 between control and Kir6.2TH-Cre mice. See also Figures S1 and S2.
Figure 2
Figure 2. Impaired BAT SNA in Kir6.2TH-Cre Mice
(A) Average daily food intake of 15-week-old male control (n = 12) and Kir6.2TH-Cre mice (n = 14) on HFD. (B) Energy expenditure corrected for lean body mass of 20-week-old male control (n = 12) and Kir6.2TH-Cre mice (n = 13) on HFD. (C) Representative hematoxylin and eosin (H&E) staining of BAT of a 20-week-old male control (upper panel) and Kir6.2TH-Cre mice (lower panel) on HFD (10× magnification in the small square). (D) Relative expression of peroxisome proliferator-activated receptor γ (Pparg [PPAR-γ]), peroxisome proliferator-activated receptor coactivator 1α (Ppargc1a [Pgc1]), uncoupling protein 1 (Ucp1), β3-adrenergic receptor (Adrb3), and cell-death-inducing DFF45-like effector protein a (Cidea) in BAT extracts from 20-week-old control (n = 6) and Kir6.2TH-Cre mice (n = 6) on HFD. (E) Immunoblot analysis (left) and quantification (right) of UCP1 protein expression in BAT of 20-week-old control and Kir6.2TH-Cre mice on HFD. α-tubulin was used as a loading control. (F) Quantification of BAT SNA of 15-week-old control (n = 5) and Kir6.2TH-Cre mice (n = 5) on HFD. (G) Adrenal SNA in 15-week-old control (n = 6) and Kir6.2TH-Cre mice (n = 6) on HFD. (H and I) Heart rate and mean arterial pressure in control (n = 4) and Kir6.2TH-Cre mice (n = 5) measured in fully awake and unrestrained mice. (J) BAT SNA response induced by i.c.v. glucose injection in control (n = 8) and Kir6.2TH-Cre mice (n = 5) on HFD. (K) A comparison of BAT SNA responses induced by i.c.v. glucose injection (an average of the last hour of recording) between control and Kir6.2TH-Cre mice. (L) Rectal temperature of 15-week-old control (n = 6) and Kir6.2TH-Cre mice (n = 6) on HFD upon cold exposure (+4°C) for 4 hr. Data are expressed as mean ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001 between control and Kir6.2TH-Cre mice. See also Figure S3.
Figure 3
Figure 3. Glucose Responses of LC Neurons in Control and Kir6.2TH-Cre Mice
(A) Perforated patch clamp recordings of LC neurons in male control (C57BL/6 and TH-Cre-negative Kir6.2 littermates) and Kir6.2TH-Cre mice. Basal firing frequency is reduced in neurons expressing the mutant Kir6.2 subunit. (B) Antibody staining of dopamine-β-hydroxylase (green) and recorded single neuron backfilled with biocytin-confirming location in the LC (left, the scale bar represents 100 μM; right, the scale bar represents 25 μM). (C) Relative reduction of action potential (AP) frequency induced by a decreased extracellular glucose concentration (5 to 3 mM) and after an increase in frequency by the application of 200 μM tolbutamide. (D) LC neurons of Kir6.2TH-Cre mice show no changes in AP frequency upon varying glucose concentrations but show increased firing upon tolbutamide application. (E) A reduction of extracellular glucose concentration (5 to 3 mM) reduces AP frequency, which is reversed by the application of 200 μM tolbutamide (upper panel, the asterisk marks current injection protocols). An increase of extracellular glucose (5 to 8 mM) concentration leads to an increase of AP frequency of control LC neurons (lower panel). (F) Relative increase of AP frequency by increasing extracellular glucose (5 to 8 mM) in control LC neurons. (G) LC neurons of Kir6.2TH-Cre mice show no changes in AP frequency upon an increase of extracellular glucose concentration but show increased firing upon tolbutamide application. (H) The number of neurons in LC that respond to an increase and decrease of extracellular glucose concentration in control and Kir6.2TH-Cre mice. Data are expressed as mean ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001. Counts (n) are expressed in the figure.
Figure 4
Figure 4. KATP Channels in the LC Control BAT Function and Energy Homeostasis
(A) A schematic representation showing the localization of AAV injection. AAV expressing either GFP or Cre was injected into the LC of Rosa26Kir6.2 mice (anteroposterior, ~5.45; mediolateral, ± 1.28; dorsoventral, 3.65). (B) Representative photomicrographs depicting GFP -immunoreactivity (green, GFP; blue, DAPI nuclear staining) from Rosa26Kir6.2 mice bilaterally injected with AAV-GFP vectors into the LC. (C) Change of body weight after bilateral LC injection of AAV-GFP (n = 6) or AAV-Cre (n = 8). Rosa26Kir6.2 mice exposed to HFD from 3 weeks of age on were injected with the respective AAV vectors at the age of 8 weeks, and body weight was followed for another 4 weeks. Change of body weight after AAV vector injection is expressed as a percentage the difference from preinjection body weight. (D) Body composition (% lean and fat mass) of mice expressing the mutant form of Kir6.2 specifically in LC (AAV-Cre) (n = 8) versus control (AAV-GFP) (n = 6) 7 weeks after AAV vector injection measured by NMR. (E) Increased epididymal fat pad weight in mice expressing the mutant form of Kir6.2 specifically in LC (AAV-Cre) (n = 7) versus control (AAV-GFP) (n = 6) 7 weeks after AAV vector injection. (F) Representative H&E staining of epididymal adipose tissue (left) of a 15-week-old male control mice (AAV-GFP; upper panel) and AAV-Cre mice (lower panel) on HFD. Quantification of mean adipocyte surface in epididymal adipose tissue of 20-week-old male control (AAV-GFP, n = 6) and AAV-Cre mice (n = 6) on HFD (right). (G) Representative H&E staining of BAT (left) of a 15-week-old male control (AAV-GFP, upper panel) and AAV-Cre (lower panel) mice on HFD (10× magnification in the small square). The relative expression of uncoupling protein 1 (Ucp1) and β3-adrenergic receptor (Adrb3) in BAT extracts of control (AAV-GFP, n = 5) and AAV-Cre mice (n = 7) on HFD (right). The expression of indicated messenger RNAs was normalized to that of Hprt, and the resultant value for each group was normalized to the expression of the target gene in control mice. (H) Time course of BAT SNA response induced by i.c.v. glucose (100 nM) in HFD-fed Rosa26Kir6.2 mice that received either AAV-GFP (n = 6) or AAV-Cre (n = 6) into the LC. (I) A comparison of BAT SNA responses after i.c.v. injection of vehicle or glucose between HFD-fed Rosa26Kir6.2 mice that received either AAV-GFP (vehicle, n = 3; glucose, n = 6) or AAV-Cre (vehicle, n = 3; glucose, n = 6) into the LC. (J) Rectal temperature of AAV-GFP (n = 5) and AAV-Cre microinjected mice (n = 8) on HFD upon cold exposure (+4°C). Data are expressed as mean ± SEM. *p < 0.05 and **p < 0.01 between control (AAV-GFP) versus AAV-Cre mice. See also Figure S4.

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