Norepinephrine Response in Brown Adipose Tissue from Newborn Rats

Hormone-induced mitochondrial fission is utilized by brown adipocytes as an amplification pathway for energy expenditure

Adrenergic stimulation of brown adipocytes (BA) induces mitochondrial uncoupling, thereby increasing energy expenditure by shifting nutrient oxidation towards thermogenesis. Here we describe that mitochondrial dynamics is a physiological regulator of adrenergically-induced changes in energy expenditure. The sympathetic neurotransmitter Norepinephrine (NE) induced complete and rapid mitochondrial fragmentation in BA, characterized by Drp1 phosphorylation and Opa1 cleavage. Mechanistically, NE-mediated Drp1 phosphorylation was dependent on Protein Kinase-A (PKA) activity, whereas Opa1 cleavage required mitochondrial depolarization mediated by FFAs released as a result of lipolysis. This change in mitochondrial architecture was observed both in primary cultures and brown adipose tissue from cold-exposed mice. Mitochondrial uncoupling induced by NE in brown adipocytes was reduced by inhibition of mitochondrial fission through transient Drp1 DN overexpression. Furthermore, forced mitochondrial fragmentation in BA through Mfn2 knock down increased the capacity of exogenous FFAs to increase energy expenditure. These results suggest that, in addition to its ability to stimulate lipolysis, NE induces energy expenditure in BA by promoting mitochondrial fragmentation. Together these data reveal that adrenergically-induced changes to mitochondrial dynamics are required for BA thermogenic activation and for the control of energy expenditure.

The nucleotide regulatory sites on the mitochondrial KATP channel face the cytosol

The mitochondrial KATP channel (mitoKATP) is richly endowed with regulatory sites for metabolites and drugs, but the topological location of these sites is unknown. Thus, it is not known whether ATP, GTP and acyl CoA esters regulate mitoKATP from the matrix or cytosolic side of the inner membrane, nor whether they all act from the same side. The experiments reported in this paper provide an unambiguous answer to these questions. Electrophysiological experiments in bilayer membranes containing purified mitoKATP showed that current is blocked asymmetrically by ATP. K+ flux experiments using proteoliposomes containing purified mitoKATP showed that mitoKATP is unipolar with respect to regulation by Mg2+, ATP, GTP, and palmitoyl CoA and that all of these ligands react on the same pole of the protein. This demonstration was made possible by the new finding that mitoKATP is 85-90% oriented inward or outward in liposomes, depending on the presence or absence of Mg2+ in the reconstitution buffer. K+ flux experiments in respiring rat liver mitochondria showed that mitoKATP was inhibited by palmitoyl CoA and activated by GTP when these agents were added to the external medium. Given that the inner membrane is impermeant to these ligands and that mitoKATP is unipolar with respect to nucleotide regulation, it follows that the regulatory sites on mitoKATP face the cytosol.

Inhibition of the mitochondrial KATP channel by long-chain acyl-CoA esters and activation by guanine nucleotides

The mitochondrial KATP channel (mitoKATP) is highly sensitive to ATP, which inhibits K+ flux with K1/2 values of 20-40 microM. This raises the question, how can mitoKATP be opened in the presence of physiological concentrations of ATP? We measured K+ flux in liposomes reconstituted with purified mitoKATP and found that guanine nucleotides are potent activators of this channel. ATP-inhibited K+ flux was completely reactivated by both GTP (K1/2 = 7 microM) and GDP (K1/2 = 140 microM). These ligands had no effect in the absence of ATP. The K1/2 for ATP inhibition exhibited quadratic dependence on [GTP] and [GDP], consistent with two binding sites for guanine nucleotides. We also found that palmitoyl-CoA and oleoyl-CoA inhibited K+ flux through reconstituted mitoKATP with K1/2 values of 260 nM and 80 nM, respectively. This inhibition was reversed by GTP (K1/2 = 232 microM) as well as by the K+ channel openers cromakalim (20 microM) and diazoxide (10 microM). Inhibition of mitoKATP by long-chain acyl-CoA esters, like that of ATP, exhibited an absolute requirement for Mg2+ ions. We propose that the open-closed state of the mitochondrial KATP channel is determined by the relative cytosolic concentrations of GTP and long-chain acyl-CoA esters.

Cation transport in mitochondria--the potassium cycle

The existence in mitochondria of separate, highly regulated pathways for K+ influx and efflux strongly implies that mitochondrial volume is subject to regulation in vivo. Volume, in turn has been shown to regulate activity of the electron transport chain. Thus, the mitochondrial K+ cycle appears to play a key signalling role in regulating cellular bioenergetics, including the metabolic fate of fatty acids. Consistent with this role, the channel is inhibited by long-chain acyl-CoA esters and activated by GTP, and these ligands interact with sites that face the cytosol. The work to be summarized shows that KATP channels from mitochondria and plasma membranes are regulated by the same biochemical and pharmacological ligands. We hypothesize that the mitochondrial KATP channel, like its counterparts in the plasma membrane, is heteromultimeric, consisting of a regulatory sulfonylurea receptor (mitoSUR) and an inward-rectifying K+ channel (mitoKIR).

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

Apparent unmasking of [3H]GDP binding in rat brown-fat mitochondria is due to mitochondrial swelling

The presence of and biochemical background for the so-called 'unmasking' phenomenon in rat brown-fat mitochondria was investigated (i.e. the apparent increase in [3H]GDP binding to the 'uncoupling' protein thermogenin, without a concomitant increase in the amount of the protein). It was found that an unmasking could be observed both 1 h after norepinephrine injection and after 1 h cold stress, provided that the rats were preacclimated to 28 degrees C. The unmasking could be observed both when a filtration method and when a centrifugation method for determination of [3H]GDP-binding capacity were used; however, the absolute values were higher with the filtration method. Based on observations of slower cytochrome-c oxidase sedimentation during centrifugation, the possibility that the matrix volume of brown-fat mitochondria isolated from warm-acclimated animals was smaller than that of cold-stressed animals was investigated with 3H2O. The cold stress increased the matrix volume from being nearly non-existent to about 1 microliter/mg. A preswelling procedure in an ionic medium could similarly increase the matrix volume in mitochondria from warm-acclimated animals but was without significant effect in the already swollen mitochondria from cold-stressed animals or from animals adapted to a lower temperature. In mitochondria from warm-acclimated animals, the ionic preswelling procedure was fully able to increase the apparent amount of GDP binding to that observed in mitochondria from cold-stressed animals, but it was practically without effect on GDP binding in mitochondria from cold-stressed animals or from animals acclimated to a lower temperature. It is concluded that the apparent 'unmasking' phenomenon, observed when the tissue is less activated than in normal control situations, is not (as hitherto anticipated) due to a specific change in thermogenin as such, but is a reflection of a general mitochondrial phenomenon.