Correlation between uncoupled ATP hydrolysis and heat production by the sarcoplasmic reticulum Ca2+-ATPase: coupling effect of fluoride

THADA Regulates the Organismal Balance between Energy Storage and Heat Production

Human susceptibility to obesity is mainly genetic, yet the underlying evolutionary drivers causing variation from person to person are not clear. One theory rationalizes that populations that have adapted to warmer climates have reduced their metabolic rates, thereby increasing their propensity to store energy. We uncover here the function of a gene that supports this theory. THADA is one of the genes most strongly selected during evolution as humans settled in different climates. We report here that THADA knockout flies are obese, hyperphagic, have reduced energy production, and are sensitive to the cold. THADA binds the sarco/ER Ca²⁺ ATPase (SERCA) and acts on it as an uncoupler. Reducing SERCA activity in THADA mutant flies rescues their obesity, pinpointing SERCA as a key effector of THADA function. In sum, this identifies THADA as a regulator of the balance between energy consumption and energy storage, which was selected during human evolution.

•

Drosophila knockouts of the conserved gene THADA are obese and hyperphagic

•

THADA knockouts produce less heat and are cold sensitive

•

THADA binds SERCA and uncouples its ATP hydrolysis from Ca²⁺ pumping

•

Reducing SERCA activity rescues the THADA loss-of-function phenotypes

1,2-Dichlorobenzene affects the formation of the phosphoenzyme stage during the catalytic cycle of the Ca(2+)-ATPase from sarcoplasmic reticulum

BACKGROUND

1,2-Dichlorobenzene (1,2-DCB) is a benzene-derived molecule with two Cl atoms that is commonly utilized in the synthesis of pesticides. 1,2-DCB can be absorbed by living creatures and its effects on naturally-occurring enzymatic systems, including the effects on Ca(2+)-ATPases, have been poorly studied. Therefore, we aimed to study the effect of 1,2-DCB on the Ca(2+)-ATPase from sarcoplasmic reticulum (SERCA), a critical regulator of intracellular Ca(2+) concentration.

RESULTS

Concentrations of 0.05-0.2 mM of 1,2-DCB were able to stimulate the hydrolytic activity of SERCA in a medium-containing Ca(2+)-ionophore. At higher concentrations (0.25-0.75 mM), 1,2-DCB inhibited the ATP hydrolysis to ~80 %. Moreover, ATP hydrolysis and Ca(2+) uptake in a medium supported by K-oxalate showed that starting at 0.05 mM,1,2-DCB was able to uncouple the ratio of hydrolysis/Ca(2+) transported. The effect of this compound on the integrity of the SR membrane loaded with Ca(2+) remained unaffected. Finally, the analysis of phosphorylation of SERCA by [γ-(32)P]ATP, starting under different conditions at 0° or 25 °C showed a reduction in the phosphoenzyme levels by 1,2-DCB, mostly at 0 °C.

CONCLUSIONS

The temperature-dependent decreased levels of phosphoenzyme by 1,2-DCB could be due to the acceleration of the dephosphorylation mechanism - E2P · Ca2 state to E2 and Pi, which explains the uncoupling of the ATP hydrolysis from the Ca(2+) transport.

Sarcolipin protein interaction with sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is distinct from phospholamban protein, and only sarcolipin can promote uncoupling of the SERCA pump

Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent data suggest that SLN could play a role in muscle thermogenesis by promoting uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 1575-1579), but the mechanistic details are unknown. To better define how binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 interaction with that of PLB in detail. The homo-bifunctional cross-linker (1,6-bismaleimidohexane) was employed to detect dynamic protein interaction during the SERCA cycle. Our studies reveal that SLN differs significantly from PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally show that PLB and SLN can bind to the same groove but interact with a different set of residues on SERCA. These data collectively suggest that SLN is functionally distinct from PLB; its ability to interact with SERCA in the presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat production.

Uncoupling of sarcoplasmic reticulum Ca²⁺-ATPase by N-arachidonoyl dopamine. Members of the endocannabinoid family as thermogenic drugs

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) plays a role in thermogenesis. The exogenous compound capsaicin increased SERCA-mediated ATP hydrolysis not coupled to Ca²⁺ transport. Here, we have sought to identify endogenous compounds that may function as SERCA uncoupling agents.

EXPERIMENTAL APPROACH

Using isolated SR vesicles from rabbits, we have screened for endogenous compounds that uncouple SERCA. We have also studied their ability to deplete cytoplasmic ATP from human skeletal muscle cells in culture.

KEY RESULTS

Studies on SR vesicles showed that the endogenous lipid metabolite N-arachidonoyl dopamine (NADA) was a potent stimulator of SERCA uncoupling. NADA stabilized an E₁-like pump conformation that had a lower dephosphorylation rate, low affinity for Ca²⁺ at the luminal sites and a specific proteinase K cleavage pattern involving protection of the C-terminal p83C fragment from further cleavage. Moreover, we found a significantly decreased cytoplasmic ATP levels following treatment of skeletal muscle cells with 100 nM NADA. This effect was dependent on the presence of glucose and abolished by pretreatment with the specific SERCA inhibitor thapsigargin, regardless of the presence of glucose.

CONCLUSIONS AND IMPLICATIONS

NADA is an endogenous molecule that may function as SERCA uncoupling agent in vivo. Members of the endocannabinoid family exert concerted actions on several Ca²⁺-handling proteins. Uncoupling of SERCA by exogenous compounds could be a novel post-mitochondrial strategy for reduction of cellular ATP levels. In addition, signalling networks leading to SERCA uncoupling can be explored to study the importance of this ion pump in pathophysiological conditions related to metabolism.

Ca2+ release to lumen from ADP-sensitive phosphoenzyme E1PCa2 without bound K+ of sarcoplasmic reticulum Ca2+-ATPase

During Ca(2+) transport by sarcoplasmic reticulum Ca(2+)-ATPase, the conformation change of ADP-sensitive phosphoenzyme (E1PCa(2)) to ADP-insensitive phosphoenzyme (E2PCa(2)) is followed by rapid Ca(2+) release into the lumen. Here, we find that in the absence of K(+), Ca(2+) release occurs considerably faster than E1PCa(2) to E2PCa(2) conformation change. Therefore, the lumenal Ca(2+) release pathway is open to some extent in the K(+)-free E1PCa(2) structure. The Ca(2+) affinity of this E1P is as high as that of the unphosphorylated ATPase (E1), indicating the Ca(2+) binding sites are not disrupted. Thus, bound K(+) stabilizes the E1PCa(2) structure with occluded Ca(2+), keeping the Ca(2+) pathway to the lumen closed. We found previously (Yamasaki, K., Wang, G., Daiho, T., Danko, S., and Suzuki, H. (2008) J. Biol. Chem. 283, 29144-29155) that the K(+) bound in E2P reduces the Ca(2+) affinity essential for achieving the high physiological Ca(2+) gradient and to fully open the lumenal Ca(2+) gate for rapid Ca(2+) release (E2PCa(2) → E2P + 2Ca(2+)). These findings show that bound K(+) is critical for stabilizing both E1PCa(2) and E2P structures, thereby contributing to the structural changes that efficiently couple phosphoenzyme processing and Ca(2+) handling.

Capsaicin stimulates uncoupled ATP hydrolysis by the sarcoplasmic reticulum calcium pump

In muscle cells the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) couples the free energy of ATP hydrolysis to pump Ca(2+) ions from the cytoplasm to the SR lumen. In addition, SERCA plays a key role in non-shivering thermogenesis through uncoupled reactions, where ATP hydrolysis takes place without active Ca(2+) translocation. Capsaicin (CPS) is a naturally occurring vanilloid, the consumption of which is linked with increased metabolic rate and core body temperature. Here we document the stimulation by CPS of the Ca(2+)-dependent ATP hydrolysis by SERCA without effects on Ca(2+) accumulation. The stimulation by CPS was significantly dependent on the presence of a Ca(2+) gradient across the SR membrane. ATP activation assays showed that the drug reduced the nucleotide affinity at the catalytic site, whereas the affinity at the regulatory site increased. Several biochemical analyses indicated that CPS stabilizes an ADP-insensitive E(2)P-related conformation that dephosphorylates at a higher rate than the control enzyme. Under conditions where uncoupled SERCA was specifically inhibited by the treatment with fluoride, low temperatures, or dimethyl sulfoxide, CPS had no stimulatory effect on ATP hydrolysis by SERCA. It is concluded that CPS stabilizes a SERCA sub-conformation where Ca(2+) is released from the phosphorylated intermediate to the cytoplasm instead of the SR lumen, increasing ATP hydrolysis not coupled with Ca(2+) transport. To the best of our knowledge CPS is the first natural drug that augments uncoupled SERCA, presumably resulting in thermogenesis. The role of CPS as a SERCA modulator is discussed.

Heat production by skeletal muscles of rats and rabbits and utilization of glucose 6-phosphate as ATP regenerative system by rats and rabbits heart Ca2+-ATPase

This report is divided in two parts. The first section shows that vesicles derived from the sarcoplasmic reticulum of rats skeletal muscle can cleave ATP at a faster rate and produce more heat that the vesicles derived from rabbit skeletal muscle. In the second part, we compared the rates of Ca(2+) transport and ATP hydrolysis by rats and rabbits heart sarcoplasmic reticulum. It is shown that the two vesicles preparations are able to use glucose 6-phosphate and hexokinase as an ATP regenerative system. The rates of Ca(2+)-uptake and ATP hydrolysis measured with glucose 6-phosphate and hexokinase is four to six times slower than that measured with phosphoenolpyruvate and pyruvate kinase as ATP regenerative system.

Thermogenic activity of Ca2+-ATPase from skeletal muscle heavy sarcoplasmic reticulum: The role of ryanodine Ca2+ channel

The sarcoplasmic reticulum Ca(2+) ATPase 1 (SERCA 1) is able to handle the energy derived from ATP hydrolysis in such a way as to determine the parcel of energy that is used for Ca(2+) transport and the fraction that is converted into heat. In this work we measured the heat production by SERCA 1 in the two sarcoplasmic reticulum (SR) fractions: the light fraction (LSR), which is enriched in SERCA and the heavy fraction (HSR), which contains both the SERCA and the ryanodine Ca(2+) channel. We verified that although HSR cleaved ATP at faster rate than LSR, the amount of heat released during ATP hydrolysis by HSR was smaller than that measured by LSR. Consequently, the amount of heat released per mol of ATP cleaved (DeltaH(cal)) by HSR was lower compared to LSR. In HSR, the addition of 5 mM Mg(2+) or ruthenium red, conditions that close the ryanodine Ca(2+) channel, promoted a decrease in the ATPase activity, but the amount of heat released during ATP hydrolysis remained practically the same. In this condition, the DeltaH(cal) values of ATP hydrolysis increased significantly. Neither Mg(2+) nor ruthenium red had effect on LSR. Thus, we conclude that heat production by SERCA 1 depends on the region of SR in which the enzyme is inserted and that in HSR, the DeltaH(cal) of ATP hydrolysis by SERCA 1 depends on whether the ryanodine Ca(2+) channel is opened or closed.

Elevated Ca2+ ATPase (SERCA2) activity in tuna hearts: comparative aspects of temperature dependence

Tunas have an extraordinary physiology including elevated metabolic rates and high cardiac performance. In some species, retention of metabolic heat warms the slow oxidative swimming muscles and visceral tissues. In all tunas, the heart functions at ambient temperature. Enhanced rates of calcium transport in tuna myocytes are associated with increased expression of proteins involved in the contraction-relaxation cycle. The cardiac SR Ca2+-ATPase (SERCA2) plays a major role during cardiac excitation-contraction (E-C) coupling. Measurements of oxalate-supported Ca2+-uptake in atrial SR vesicles isolated from four species of tunas indicate that bluefin have at least two fold higher Ca2+-uptake than all other tunas examined between 5 and 30 degrees C. The highest atrial Ca2+-uptake was measured in bluefin tuna at 30 degrees C (23.32+/-1.58 nmol Ca2+/mg/min). Differences among tunas in the temperature dependency of Ca2+-uptake were similar for ATP hydrolysis. Western blot analysis revealed a significant increase in SERCA2 content associated with higher Ca2+ uptake rates in the atrial tissues of bluefin tuna and similar RyR expression across species. We propose that the expression of EC coupling proteins in cardiac myocytes, and the higher rates of SERCA2 activity are an important evolutionary step for the maintenance of higher heart rates and endothermy in bluefin tuna.

Role of sarco/endoplasmic reticulum Ca(2+)-ATPase in thermogenesis

Enzymes are able to handle the energy derived from the hydrolysis of phosphate compounds in such a way as to determine the parcel that is used for work and the fraction that is converted into heat. The sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCA) is a family of membrane-bound ATPases that are able to transport Ca(2+) ion across the membrane using the chemical energy derived from ATP hydrolysis. The heat released during ATP hydrolysis by SERCA may vary from 10 up to 30 kcal/mol depending on the SERCA isoform used and on whether or not a Ca(2+) gradient is formed across the membrane. Drugs such as heparin, dimethyl sulfoxide and the platelet-activating factor (PAF) are able to modify the fraction of the chemical energy released during ATP hydrolysis that is used for Ca(2+) transport and the fraction that is dissipated in the surrounding medium as heat. The thyroid hormone 3,5,3'-triiodo L: -thyronine (T(3)) regulates the expression and function of the thermogenic SERCA isoforms. Modulation of heat production by SERCA might be one of the mechanisms involved in the increased thermogenesis found in hyperthyroidism.

The thermogenic activity of rat brown adipose tissue and rabbit white muscle Ca2+-ATPase

The Ca2+-ATPase (SERCA) found in vesicles derived from the sarco/endoplasmic reticulum vesicles of rats brown adipose tissue and rabbit white muscle were identified by gel electrophoresis, Western blot, electron microscopy and immunolabeling with gold particles. In both tissues, the isoform found was SERCA 1. The Ca2+ affinity of the fat SERCA 1 was different from the muscle isoform. The degree of uncoupling is estimated measuring the ratio between Ca2+ transport and ATP cleaved. In brown fat vesicles the degree of uncoupling varied depending on the Ca2+ concentration of the medium. This was not observed in vesicles derived from muscle. At all Ca2+ concentrations tested, the uncoupling was not related to Ca2+ leakage from the membrane and was far more pronounced in fat than in muscle vesicle. When a Ca2+ gradient was formed across the vesicles membrane the heat released during ATP hydrolysis varied between 22 and 26 Kcal/mol in both fat and muscle vesicles but in the absence of a gradient the heat released was 17 Kcal/mol in fat and 12 Kcal/mol in muscle. The data reported indicate that the SERCA 1 of brown adipocytes is far more thermogenic than the white muscle SERCA 1, and suggest that, in addition to storing Ca2+ inside the endoplasmic reticulum, the SERCA 1 may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.

Heat of PPi Hydrolysis Varies Depending on the Enzyme Used

With yeast-soluble inorganic pyrophosphatase, the heat released during PP(i) hydrolysis was -6.3 kcal/mol regardless of the KCl concentration in the medium. With the membrane-bound pyrophosphatase of corn vacuoles, the heat released varies between -23.5 and -7.5 kcal/mol depending on the KCl concentration in the medium and whether or not a H(+) gradient is formed across the vacuole membranes. The data support the proposal that enzymes are able to handle the energy derived from phosphate compound hydrolysis in such a way as to determine the parcel that is used for work and the fraction that is converted into heat.

Hyperthyroidism increases the uncoupled ATPase activity and heat production by the sarcoplasmic reticulum Ca2+-ATPase

The sarcoplasmic reticulum Ca2+-ATPase is able to modulate the distribution of energy released during ATP hydrolysis, so that a portion of energy is used for Ca2+ transport (coupled ATPase activity) and a portion is converted into heat (uncoupled ATPase activity). In this report it is shown that T4 administration to rabbits promotes an increase in the rates of both the uncoupled ATPase activity and heat production in sarcoplasmic reticulum vesicles, and that the degree of activation varies depending on the muscle type used. In white muscles hyperthyroidism promotes a 0.8-fold increase of the uncoupled ATPase activity and in red muscle a 4-fold increase. The yield of vesicles from hyperthyroid muscles is 3-4-fold larger than that obtained from normal muscles; thus the rate of heat production by the Ca2+-ATPase expressed in terms of g of muscle in hyperthyroidism is increased by a factor of 3.6 in white muscles and 12.0 in red muscles. The data presented suggest that the Ca2+-ATPase uncoupled activity may represent one of the heat sources that contributes to the enhanced thermogenesis noted in hyperthyroidism.

Brown adipose tissue Ca2+-ATPase: uncoupled ATP hydrolysis and thermogenic activity

In this report a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) was identified in rats brown adipose tissue. Electrophoretic analysis of brown fat microssomal protein yields a 110-kDa band that is reactive to SERCA 1 antibody but is not reactive to SERCA 2 antibodies. Nevertheless, the kinetics properties of the brown fat SERCA differ from the skeletal muscle SERCA 1 inasmuch they manifest a different Ca2+ affinity and a much higher degree of ATPase/Ca2+ uncoupling. A SERCA enzyme is not found in white fat. Fatty acids promoted Ca2+ leakage from brown fat vesicles. The heat released during ATP hydrolysis was -24.7 kcal/mol when a Ca2+ gradient was formed across the vesicles membrane and -14.4 kcal/mol in the absence of a gradient. The data reported suggest that in addition to storing Ca2+ inside the endoplasmic reticulum, the Ca2+-ATPase may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.

Catalysis, evolution and life

Living organisms are unique in their ability to generate and replicate ordered systems from disordered components. Generation of order, replication of the individual, and evolution of the species all depend on the successful utilization of external energy derived from chemicals and light. The information for reproduction is encoded in nucleic acids, but evolution depends on a limited variability in replication, and proceeds through the selection of individuals with altered biochemistry. Essentially all biochemistry is catalyzed; therefore, altered biochemistry implies altered or new catalysts. In that sense catalysis is the medium of evolution. We propose that a basic property of enzymes, at least as fundamental as reaction rate enhancement, is to adjust the reaction path by altering and eventually optimizing the reversible interchange of chemical, electrical and mechanical energy among themselves and their reactants.

The thermogenic function of the sarcoplasmic reticulum Ca2+-ATPase of normal and hyperthyroid rabbit

After formation of a Ca(2+) gradient, the amount of heat released during the hydrolysis of each mol of ATP cleaved (DeltaH(cal)) varies depending on the Ca(2+)-ATPase isoform expressed by the muscle cell. In vesicles derived from the sarcoplasmic reticulum of white muscle (SERCA 1) most of the ATP cleaved is not coupled to Ca(2+) transport, and the DeltaH(cal) varies between -20 and -22 kcal/mol. In contrast, in vesicles derived from red muscle (SERCA 2a) the hydrolysis of ATP is coupled with Ca(2+) transport, and the DeltaH(cal) varies between -12 and -14 kcal/mol. Hyperthyroidism increases the rate of heat production by the Ca(2+)-ATPase fourfold in white muscle and 40-fold in red muscle. In hyperthyroid rabbits, the amount of sarcoplasmic reticulum protein recovered from white and red muscle is four- to fivefold greater than that obtained from control rabbits. Hyperthyroid red muscle expresses SERCA 1, and the vesicles derived from these muscle hydrolyze ATP through a catalytic route that is not coupled to Ca(2+) transport, thus increasing the amount of heat released during ATP hydrolysis, the DeltaH(cal) varying between -20 and -22 kcal/mol.

Uncoupled ATP hydrolysis and thermogenic activity of the sarcoplasmic reticulum Ca2+-ATPase: coupling effects of dimethyl sulfoxide and low temperature

The sarcoplasmic reticulum Ca(2+)-ATPase transports Ca(2+) using the energy derived from ATP hydrolysis. During catalysis, part of the energy is used to translocate Ca(2+) across the membrane, and part is dissipated as heat. At 35 degrees C the heat released during the hydrolysis of each ATP molecule varies depending on the formation of a Ca(2+) gradient across the membrane. With leaky vesicles (no gradient) the heat released varies between 9 and 12 kcal/mol of ATP cleaved, and with intact vesicles (gradient), the heat released increases to 20-24 kcal/mol of ATP. After Ca(2+) accumulation, 82% of the Ca(2+)-ATPase activity is not coupled to Ca(2+) transport, and the ratio between Ca(2+) transported and ATP cleaved is 0.3. The addition of 20% dimethyl sulfoxide (v/v) to the medium or decreasing the temperature from 35 to 20 degrees C abolishes the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient. This is accompanied by a simultaneous inhibition of the uncoupled ATPase activity and an increase of the Ca(2+)/ATP ratio from 0.3 to 1.3-1.4. It is concluded that the uncoupled Ca(2+)-ATPase is responsible for both the low Ca(2+)/ATP ratio measured during transport and the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient.