Impairment of trophic response of brown fat to cold in guanethidine-treated rats

Sympathetic innervation suppresses the autophagic-lysosomal system in brown adipose tissue under basal and cold-stimulated conditions

The sympathetic nervous system (SNS) activates cAMP signaling and promotes trophic effects on brown adipose tissue (BAT) through poorly understood mechanisms. Because norepinephrine has been found to induce antiproteolytic effects on muscle and heart, we hypothesized that the SNS could inhibit autophagy in interscapular BAT (IBAT). Here, we describe that selective sympathetic denervation of rat IBAT kept at 25°C induced atrophy, and in parallel dephosphorylated forkhead box class O (FoxO), and increased cathepsin activity, autophagic flux, autophagosome formation, and expression of autophagy-related genes. Conversely, cold stimulus (4°C) for up to 72 h induced thermogenesis and IBAT hypertrophy, an anabolic effect that was associated with inhibition of cathepsin activity, autophagic flux, and autophagosome formation. These effects were abrogated by sympathetic denervation, which also upregulated Gabarapl1 mRNA. In addition, the cold-driven sympathetic activation stimulated the mechanistic target of rapamycin (mTOR) pathway, leading to the enhancement of protein synthesis, evaluated in vivo by puromycin incorporation, and to the inhibitory phosphorylation of Unc51-like kinase-1, a key protein in the initiation of autophagy. This coincided with a higher content of exchange protein-1 directly activated by cAMP (Epac1), a cAMP effector, and phosphorylation of Akt at Thr³⁰⁸, all these effects being abolished by denervation. Systemic treatment with norepinephrine for 72 h mimicked most of the cold effects on IBAT. These data suggest that the noradrenergic sympathetic inputs to IBAT restrain basal autophagy via suppression of FoxO and, in the setting of cold, stimulate protein synthesis via the Epac/Akt/mTOR-dependent pathway and suppress the autophagosome formation, probably through posttranscriptional mechanisms.NEW & NOTEWORTHY The underlying mechanisms related to the anabolic role of sympathetic innervation on brown adipose tissue (BAT) are unclear. We show that sympathetic denervation activates autophagic-lysosomal degradation, leading to a loss of mitochondrial proteins and BAT atrophy. Conversely, cold-driven sympathetic activation suppresses autophagy and stimulates protein synthesis, leading to BAT hypertrophy. Given its high-potential capacity for heat production, understanding the mechanisms that contribute to BAT mass is important to optimize chances of survival for endotherms in cold ambients.

Analysis and measurement of the sympathetic and sensory innervation of white and brown adipose tissue

Here, we provide a detailed account of how to denervate white and brown adipose tissue (WAT and BAT) and how to measure sympathetic nervous system (SNS) activity to these and other tissues neurochemically. The brain controls many of the functions of WAT and BAT via the SNS innervation of the tissues, especially lipolysis and thermogenesis, respectively. There is no clearly demonstrated parasympathetic innervation of WAT or the major interscapular BAT (IBAT) depot. WAT and BAT communicate with the brain neurally via sensory nerves. We detail the surgical denervation (eliminating both innervations) of several WAT pads and IBAT. We also detail more selective chemical denervation of the SNS innervation via intra-WAT/IBAT 6-hydroxy-dopamine (a catecholaminergic neurotoxin) injections and selective chemical sensory denervation via intra-WAT/IBAT capsaicin (a sensory nerve neurotoxin) injections. Verifications of the denervations are provided (HPLC-EC detection for SNS, ELIA for calcitonin gene-related peptide (proven sensory nerve marker)). Finally, assessment of the SNS drive to WAT/BAT or other tissues is described using the alpha-methyl-para-tyrosine method combined with HPLC-EC, a direct neurochemical measure of SNS activity. These methods have proven useful for us and for other investigators interested in innervation of adipose tissues. The chemical denervation approach has been extended to nonadipose tissues as well.

Cold-induced alterations of phospholipid fatty acyl composition in brown adipose tissue mitochondria are independent of uncoupling protein-1

The recruitment process induced by acclimation of mammals to cold includes a marked alteration in the acyl composition of the phospholipids of mitochondria from brown adipose tissue: increases in 18:0, 18:2(n-6), and 20:4(n-6) and decreases in 16:0, 16:1, 18:1, and 22:6(n-3). A basic question is whether these alterations are caused by changes in the concentration of uncoupling protein-1 (UCP1) or the thermogenesis it mediates-implying that they are secondary effects-or whether they are an integrated, independent part of the recruitment process. This question was addressed here using wild-type and UCP1-ablated C57BL/6 mice acclimated to 24 degrees C or 4 degrees C. In wild-type mice, the phospholipid fatty acyl composition of mitochondria from brown adipose tissue showed the changes in response to cold that were expected from observations in other species and strains. The changes were specific, as different changes occurred in skeletal muscle mitochondria. In UCP1-ablated mice, cold acclimation induced acyl alterations in brown adipose tissue that were qualitatively identical and quantitatively similar to those in wild-type mice. Therefore, neither the increased content of UCP1 nor mitochondrial uncoupling altered the effect of cold on acyl composition. Cold acclimation in wild-type mice had little effect on phospholipid acyl composition in muscle mitochondria, but cold-acclimation in UCP1-ablated mice caused significant alterations, probably due to sustained shivering. Thus, the alterations in brown adipose tissue phospholipid acyl composition are revealed to be an independent part of the recruitment process, and their functional significance for thermogenesis should be elucidated.

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

The control of UCP1 is dissociated from that of PGC-1alpha or of mitochondriogenesis as revealed by a study using beta-less mouse brown adipocytes in culture

In rodent brown adipose tissue, the beta-adrenergic signaling is believed, by an action on PGC-1alpha, to control UCP1 expression and mitochondriogenesis. We addressed this hypothesis using beta(1)/beta(2)/beta(3)-adrenoceptor knockout (beta-less) brown adipocytes in primary culture. In these cells: (a) proliferation and differentiation into multilocular cells were normal; (b) UCP1 mRNA expression was dramatically decreased (by 93%), whereas PGC-1alpha and mtTFA mRNA expressions were not; (c) UCP1, PGC-1alpha and COX IV protein expressions were decreased by 97%, 62% and 22%, respectively. Altogether the data show a dissociation between the control of UCP1, which is mostly beta-adrenoceptor-dependent and that of PGC-1alpha and of mitochondriogenesis which are not.

Chemical sympathectomy alters food intake and thermogenic responses to catecholamines in rats

It has been suggested that the sympathetic nervous system contributes to the short-term control of feeding. The adrenergic innervation of some splanchnic organs seems to be especially involved in such processes, since catecholamines reduce feeding only when injected intraperitoneally or intraportally. In this work, the effects of neonatal sympathetic denervation with guanethidine (Gnt) upon food intake were assessed in adult rats. Gnt-treated male rats had lower body weight gain. The hypophagic response to intraperitoneal (ip) norepinephrine was 70% higher in Gnt-treated animals as compared to controls (P < 0.05); that of epinephrine (E) by 33% (P < 0.05) and that of isoproterenol was not significantly modified. As in normal rats, the hypophagic effect was much stronger after ip than after intramuscular (im) administration (P < 0.05). On the other hand, resting oxygen consumption (VO2) was consistently lower in denervated animals. Ip E administration did not modify VO2, while im E caused increased motor activity and VO2 (P < 0.05). In contrast to control rats, the respiratory exchange ratio in ad libitum fed Gnt rats did not decrease after Ip E administration, suggesting a lack of effect upon lipid mobilization. The lower rate of body weight gain induced by neonatal Gnt sympathectomy might be due to lower daily food intake possibly related, in part, to the sensitization of the alpha-adrenergic porto-hepatic response to endogenous catecholamines. Compared with controls, Gnt-treated rats also showed a limited thermogenic capacity not related to feeding, and a greater degree of carbohydrate oxidation, possibly due to a defect in E-induced lipolysis, which is beta-adrenergic.

Rapid increase of mitochondrial uncoupling protein and its mRNA in stimulated brown adipose tissue

The increase in mitochondrial uncoupling protein in brown adipose tissue during acute stimulation by exposure of animals to cold was examined. Uncoupling protein level increased during the first hours of tissue stimulation. Use of a cDNA probe shows that synthesis of uncoupling protein mRNA was quickly stimulated. Animals treated with propranolol exhibited neither increase in uncoupling protein mRNA nor increase in the protein itself.

Quantitative evaluations of gap junctions in old rat brown adipose tissue after cold acclimation: a freeze-fracture and ultra-structural study

The morphological and functional modifications of brown adipose tissue (BAT), the tissue responsible for non-shivering thermogenesis, are well established during the phases of active stimulation (i.e. neonatal period and cold acclimation) in young animals. The 'active' brown adipocytes are filled with numerous small lipid vacuoles and large mitochondria packed with cristae rich in the protonophore uncoupling protein (UCP), whereas the 'quiescent' cell shows larger, confluent vacuoles and smaller mitochondria with rarefied cristae poor of the uncoupling protein. It is well known from literature that also gap junctions (gjs), responsible for the electrical coupling among adjacent adipocytes, modify their size following the physiological stimulus in young animals. This is in agreement with the morphology of the functionally active brown adipocyte, i.e. the multilocular, UCP-positive cell. Although the presence of the BAT in old animals is well documented, less is known about its reactivity to physiological stimuli. The present work demonstrates that after cold acclimation brown adipocytes of old rats (2 years) change their ultrastructure in a similar way as in young rats. A quantitative analysis of gap junction areas on replicas obtained by the freeze fracture technique, showed that gj increase in size (mean area 53.2 vs 110.4 x 10(-3) microns2, p = 0.003). All these morphological modifications are quite similar to those observed in BAT of young and young adult rats, supporting the hypothesis of a physiological role of brown adipose tissue at every age.

Metabolic regulation of in vivo myocardial contractile function: multiparameter analysis

To gain insight into the mechanisms of myocardial regulation as it relates to the interaction of mechanical and metabolic function and perfusion, intact animal models were instrumented for routine physiological measurements of mechanical function and for measurements of metabolism (31P NMR, NADH fluorescence (redox state)) and perfusion (2H NMR and Laser doppler techniques). These techniques were applied to canine and cat models of volume and/or pressure loading, hypoxia, ischemia and cardiomyopathic states. Data generated using these techniques indicate that myocardial bioenergetic function is quite stable under most loading conditions as long as the heart is not ischemic. In addition, these data indicate that there is no universal regulator and that different biochemical regulators appear to mediate stable function under different physiological and pathophysiological conditions: for example; during hypoxia, NADH redox state appears to play a regulatory role; and in pressure loading, ADP, phosphorylation potential and free energy of ATP hydrolysis as well as NADH redox state appear to be regulatory.

Meal associated changes in brown fat thermogenesis and glycogen

Data indicate a close association between a decrease in feeding-induced brown adipose tissue (BAT) thermogenesis and an increase in food consumption. The present study examines the hypothesis that feeding-induced BAT thermogenesis, or feeding-induced changes in BAT glycogen, a mobile form of energy store and a correlate of BAT thermogenesis, may modulate feeding behavior. We report that propranolol, which completely abolished meal-induced BAT thermogenesis, did not evoke intake of a larger meal. Though BAT glycogen concentration is a sensitive measure of the state of feeding, on a meal to meal basis it does not correlate with hunger and satiety. Hence the hypothesis is not supported by the current data. We also report that meal-induced BAT hypertrophy and glycogen deposition can be dissociated from meal-induced BAT thermogenesis.

Effect of noradrenaline chronic administration on brown fat phospholipids

Chronic cold exposure of rats (9 days at 5 degrees C) induces an alteration of the fatty acid composition of phospholipids in brown adipose tissue. The alteration is due to an increase of the unsaturation degree of these lipids. The phenomenon can be reproduced by 10(-7) mole. h-1 administration of noradrenaline for 9 days in rats kept at 25 degrees C. Thus, phospholipid alteration in brown fat of cold exposed rats is most probably a consequence of the increase of sympathetic tone which occurs in this tissue during exposure to cold.

Loss of brown adipose tissue uncoupling protein mRNA on deacclimation of cold-exposed rats

The effect of environmental temperature on the level of uncoupling protein mRNA from rat brown adipose tissue was examined using a cDNA probe. A 4.4 fold increase in the mRNA level was observed after 1 day exposure of rats to 6 degrees C, which was followed by a slow loss with longer times of exposure. When rats were returned to a thermoneutral environment, there was a dramatic loss of uncoupling protein mRNA within 1 day. Comparison wih poly(A)+ RNA levels suggest that the response to temperature is specific for uncoupling protein mRNA.

A preadipocyte clonal line from mouse brown adipose tissue. Short- and long-term responses to insulin and beta-adrenergics

A clonal cell line has been established from the interscapular brown adipose tissue (BAT) of the C57 BL/6J +/+ mouse. The line, designated BFC-1, is aneuploid and exhibits both morphological and biochemical properties characteristics of mature adipocytes. Adipose conversion begins after confluence and is accompanied by an early emergence of lipoprotein lipase; a later emergence of glycerol-3-phosphate dehydrogenase and acid: CoA ligase; an increase in the average triglyceride content. Adipose conversion, estimated by activities of enzyme markers, is enhanced at any given time by the continuous presence in the culture medium of insulin and triiodothyronine, both within their physiological range of concentrations. In addition to both hormones, chronic exposure of confluent cells to beta-adrenergics brings similar long-term effects on adipose conversion. The uptake of labelled 2-deoxyglucose by differentiated BFC-1 cells is stimulated by insulin; the half-maximum effect is observed at 1 nM insulin. Differentiated BFC-1 cells, in which endogenous triglycerides have been prelabelled on the fatty acid moiety, do respond to beta-adrenergics by releasing radioactive fatty acids. The agonist potency order and the EC50 value for each agonist are BRL 37344 (0.5 nM) greater than isoproterenol (1.5 nM) greater than norepinephrine (3 nM) greater than epinephrine (7 nM) greater than salbutamol (15 nM). The half-maximally and maximally effective concentrations of corticotropin to stimulate lipolysis are found to be 4 and 100 nM, respectively. The lipolytic response to isoproterenol is counteracted by prior addition of insulin or simultaneous addition of propranolol. Parallel studies performed on Ob17 cells, a clonal line established from mouse white adipose tissue (Négrel et al., Proc natl acad sci US 75 (1978) 6054), show that the agonist potency order and the EC50 value for each agonist are BRL 37344 (3 nM) greater than isoproterenol (10 nM) greater than norepinephrine (20 nM) greater than epinephrine (40 nM). Thus both BFC-1 cells and Ob17 cells show an atypical beta-adrenoreceptor similar to that described in rat adipocytes (Arch et al., Nature 309 (1984) 163), but the sensitivity of BFC-1 cells toward beta-agonists is found to be 6-fold higher than that of Ob17 cells. Thus the BFC-1 line represents a useful model for the study of short- and long-term responses to beta-adrenergics.

Influence of sympathectomy on body weight of rats given chow or supermarket diets

Rats that were neonatally sympathectomized with guanethidine (GUA) and given access to a four-component supermarket diet (SD) when either adolescent or adult gained weight at the same rate as controls. Adrenodemedullation (MDL) of adults, either alone or in combination with GUA treatment, also failed to influence the development of dietary obesity. Neonatal sympathectomy induced small changes in caloric consumption and diet preferences. These results question the notion that the sympathetic nervous system (SNS) is an essential contributor to the control of body weight and the development of dietary obesity.

Molecular approach to thermogenesis in brown adipose tissue: cDNA cloning of the mitochondrial uncoupling protein

The uncoupling protein (UCP) of mammalian brown fat is a specialized and unique component responsible for energy dissipation as heat. Translation and immunoprecipitation from sucrose-fractionated mRNA indicated that the mRNA of UCP sedimented at 14-16 S. A recombinant cDNA library prepared from mRNA of thermoactive brown fat enriched for UCP mRNA has been constructed and cloned in Escherichia coli. Recombinant plasmids were screened by differential colony hybridization to a cDNA probe complementary to poly(A)+ RNA isolated from thermogenic or from weakly thermogenic brown fat. Several differentially hybridizing plasmids were shown to contain UCP cDNA sequences by their ability to select a mRNA coding for an in vitro translation product that was immunoprecipitable with antibodies against UCP. Blot hybridization of brown fat mRNA to a 32P-labeled UCP cDNA probe revealed two major species of mRNA (15S and 18S). As compared to non-thermogenic tissue, a strikingly increased hybridization to the probe was observed with brown fat mRNA from thermoactive tissue. Moreover, hybridization was observed with RNA of brown adipose tissue from rat, hamster, or mouse but not with RNA from rat or mouse liver.

Guanethidine sympathectomy does not prevent meal-induced increases in the weight or oxygen consumption of brown fat

The interscapular brown adipose tissue (BAT) of adult rats that were neonatally sympathectomized with guanethidine (GUA) consumed less oxygen but weighed the same as BAT from intact controls. In response to a 2-hr mixed-constituent meal, BAT from sympathectomized and control rats showed similar increases in oxygen uptake and weight. These data suggest that some functions of BAT can be maintained even without sympathetic stimulation.

Quantitative evaluation of gap junctions in rat brown adipose tissue after cold acclimation

The decrease in the metabolic capacity of rat brown adipose tissue during the late postnatal period can be reversed by cold acclimation of the animals. In order to find out whether a parallel decrease in capability for intercellular communication observed during this period is also reversed by cold acclimation, gap junction size and number per unit area of cell surface have been quantified in freeze-fracture replicas; cell diameters have been measured in semi-thin sections. It was found that the specific number of gap junctions remains unchanged during cold acclimation. However, the mean gap junction size increases by 75% and the ratio of gap junctional area per cell volume, an index for intercellular exchange capacity, is doubled. This result illustrates further the parallelism between metabolic capacity and cell communication in brown fat.

Prolonged beta-adrenoceptor stimulation increases the amount of GDP-binding protein in brown adipose tissue mitochondria

Cold acclimation increases the thermogenic capacity of brown adipose tissue (BAT) and this is associated with both a general hypertrophy of the tissue and a selective increase in the proportion of a specific 32,000 molecular weight, GDP-binding protein in the mitochondrial membrane. This protein regulates the activity of the major thermogenic pathway of BAT. Although noradrenaline mediates the acute thermogenic response to cold, repeated injections of this hormone fail to mimic cold acclimation in that they do not increase the proportion of 32,000 molecular weight (32K) protein. However, noradrenaline is rapidly metabolised in vivo and effective levels would only be maintained over a short period of time following administration of the hormone. We have therefore investigated the effects of two long-acting sympathomimetic compounds: fenoterol, a beta-adrenoceptor agonist, and ephedrine, which causes release of noradrenaline from nerve terminals. Chronic treatment with both of these compounds increased the GDP-binding ability of BAT mitochondria and the proportion of 32K protein as measured by gel electrophoresis. We suggest that prolonged stimulation of beta-adrenoceptors, probably in BAT, but possibly also in other tissues, is sufficient to produce a selective increase in GDP-binding protein in BAT, and it is unnecessary to invoke any factor that is not a consequence of beta-adrenoceptor stimulation to explain the effect of cold.

Noradrenaline controls the concentration of the uncoupling protein in brown adipose tissue

The importance of noradrenaline in the control of the level of the uncoupling protein responsible for the high thermogenic capacity of brown adipose tissue mitochondria was examined. It was observed that chronic infusion of noradrenaline through mini-osmotic pumps increased the mitochondrial concentration of this uncoupling protein to the same extent as chronic exposure to cold.

The regulation of the proton conductance of brown fat mitochondria. Identification of functional and non-functional nucleotide-binding sites

The binding of purine nucleotides to intact brown fat mitochondria is re-examined. In addition to the previously reported high affinity binding site, a low-affinity site is found, which requires several minutes to saturate. Only the high affinity site is functional in regulating the proton and halide permeabilities of the mitochondria. The low affinity site can introduce errors in the use of nucleotide binding to quantitate the Mr 32000 uncoupling protein unique to these mitochondria.