Sphingosylphosphocholine modulates the ryanodine receptor/calcium-release channel of cardiac sarcoplasmic reticulum membranes

Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.

Molecular cloning and characterization of SCaMPER, a sphingolipid Ca2+ release-mediating protein from endoplasmic reticulum

Release of Ca2+ stored in endoplasmic reticulum is a ubiquitous mechanism involved in cellular signal transduction, proliferation, and apoptosis. Recently, sphingolipid metabolites have been recognized as mediators of intracellular Ca2+ release, through their action at a previously undescribed intracellular Ca2+ channel. Here we describe the molecular cloning and characterization of a protein that causes the expression of sphingosyl-phosphocholine-mediated Ca2+ release when its complementary RNA is injected into Xenopus oocytes. SCaMPER (for sphingolipid Ca2+ release-mediating protein of endoplasmic reticulum) is an 181 amino acid protein with two putative membrane-spanning domains. SCaMPER is incorporated into microsomes upon expression in SO cells or after translation in vitro. It mediates Ca2+ release at 4 degrees C as well as 22 degrees C, consistent with having ion channel function. The EC50 for Ca2+ release from Xenopus oocytes is 40 microM, similar to sphingosyl-phosphocholine-mediated Ca2+ release from permeabilized mammalian cells. Because Ca2+ release is not blocked by ryanodine or La3+, the activity described here is distinct from the Ca2+ release activity of the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor. The properties of SCaMPER are identical to those of the sphingolipid-gated Ca2+ channel that we have previously described. These findings suggest that SCaMPER is a sphingolipid-gated Ca2+-permeable channel and support its role as a mediator of this pathway for intracellular Ca2+ signal transduction.

Green fluorescent fusion proteins: powerful tools for monitoring protein expression in live zebrafish embryos

The recent development of transgenic technology in zebrafish has opened an exciting new avenue in which to explore vertebrate development. However, as in other species, the inability to easily identify live transgenic fish severely limits the potential of this promising technology. To determine whether the recently described green fluorescent protein (GFP) might provide a convenient live staining method in zebrafish, we constructed a glutathione S-transferase/GFP fusion protein (GST-GFP). GST-GFP cRNA, when injected into individual blastomeres of early zebrafish embryos, resulted in the rapid development (3 hr) of easily detectable green fluorescence which persisted for up to 4 days. GFP fluorescence was restricted to progeny of the injected cell and appeared to have no adverse effects on embryonic development despite widespread expression. Our findings demonstrate that GFP fusion proteins will provide a simple yet powerful means of monitoring production of heterologous proteins in live zebrafish.

Regulation of cell signaling by the cytoplasmic domains of the heat-stable enterotoxin receptor: identification of autoinhibitory and activating motifs

Infection with enterotoxigenic Escherichia coli is a leading cause of traveler's diarrhea. Many enterotoxigenic E. coli strains produce heat-stable enterotoxin (ST), a peptide that binds to the intestinal receptor guanylyl cyclase C known as STaR. The toxin-receptor interaction elevates intracellular cGMP, which then activates apical chloride secretion, resulting in secretory diarrhea. In this report, we examine how the intracellular domains of STaR participate in the propagation and regulation of signaling. We show that STaR exists as an oligomer in both the presence and the absence of toxin. We also demonstrate that deletion of the intracellular kinase-homology domain produces a constitutively active mutant, suggesting that this domain subserves an autoinhibitory function. Finally, we constructed a point mutant within a highly conserved region of the cyclase domain that completely inactivates the catalytic activity of guanylyl cyclase. Cotransfection of this point mutant with wild-type receptor causes a dominant-negative effect on receptor activation. This suggests that interaction of receptor subunits is required for toxin-induced activation and that the cyclase domain is involved in this essential interaction. We propose that the binding of ST to STaR promotes a conformational change across the cell membrane. This removes the inhibitory effects of the kinase-homology domain and promotes an interaction between cyclase domains that leads to receptor activation. The data suggest a paradigm of signal transduction that may also be relevant to other members of the guanylyl cyclase receptor family.

Sphingolipid-gated Ca2+ release from intracellular stores of endothelial cells is mediated by a novel Ca(2+)-permeable channel

Sphingolipid-gated Ca2+ signaling is mediated through Ca(2+)-permeable channels. In this report, we characterize the properties of the channel in a human endothelial cell line (EA.hy926). Ca2+ release from intracellular stores is not antagonized by nifedipine, omega conotoxin G-VIa, or heparin. To further characterize the molecular properties of the channel, we developed a novel assay to directly measure efflux of Ca2+ from intracellular stores of permeabilized Xenopus oocytes. Following size fractionation by sucrose gradient, poly(A)+ RNA from EA.hy926 cells is microinjected into oocytes of Xenopus laevis. We find that the mRNA encoding Ca2+ release activity is approximately 1.5-2.0 kilobases in length. The sphingolipid-gated Ca(2+)-permeable channel is thus likely to be a novel Ca(2+)-permeable channel distinct from other characterized intracellular Ca2+ channels such as the ryanodyne receptor and the inositol 1,4,5-trisphosphate receptor. The method described here provides a new approach to further characterizing this channel and other intracellular Ca2+ channels.

Two inositol 1,4,5-trisphosphate binding sites in rat basophilic leukemia cells: relationship between receptor occupancy and calcium release

Quantal calcium release is a novel paradigm for second messenger signal transduction which provides spatial and temporal control of calcium release from intracellular stores by inositol 1,4,5-trisphosphate (InsP3). We have proposed a mechanism to account for this phenomenon [Kindman, L. A., & Meyer, T. (1993) Biochemistry 32, 1270-1277], which hypothesized the existence of five channels, each with a different affinity for InsP3. As a direct test of this hypothesis, InsP3 binding to microsomes from RBL cells was examined under conditions similar to those used for calcium release. Scatchard analyses performed under a variety of conditions indicates the presence of high affinity (KD = 0.9 +/- 0.3 nM) and low affinity (KD = 47 +/- 5 nM) InsP3 binding sites. The low affinity sites are more prevalent, constituting 82 +/- 5% of the total. Both sites are identified in the presence and absence of MgATP. Moreover, both sites are selective for InsP3 over InsP4, through high concentrations of InsP4 displace InsP3 from each site (with inhibition constants of 16 and 267 nM InsP4, respectively). The relative abundance of the two InsP3 binding sites is Ca2+ dependent. An increase in Ca2+ from 0.1 to 0.5 microM results in the apparent conversion of a portion of the low affinity sites into high affinity sites into high affinity sites. Ca2+ (0.5 microM) also increased the KD of the low affinity InsP3 binding site. Given the presence of both high and low affinity InsP3 binding sites, two simple mathematical models describing both the kinetics of calcium release and quantal calcium release from RBL cells were developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a novel intracellular sphingolipid-gated Ca(2+)-permeable channel from rat basophilic leukemia cells

Sphingolipids stimulate the release of Ca2+ from intracellular stores. However, the mechanism by which this process occurs has not been characterized. Through single-channel recording from microsomes incorporated into planar lipid bilayers, we describe a novel channel that gates Ba2+ in response to sphingosylphosphorylcholine. The channel is both ligand-gated and voltage-modulated. Maximal open probability is observed between -10 and -20 mV and has a relatively high conductance (160 picosiemens with 53 mM Ba2+). We also observe that Ca2+ efflux from permeabilized rat basophilic leukemia cells is not antagonized by heparin, La3+, Ni2+, nifedipine, or omega-conotoxin GVIa. The sphingolipid-gated Ca(2+)-permeable channel is therefore a new member of the Ca(2+)-permeable, ligand-gated channel family.

Abnormalities of pulmonary function in patients with congestive heart failure, and reversal with ipratropium bromide

Patients with congestive heart failure (CHF) have baseline restrictive and obstructive abnormalities in pulmonary function. Thus, improvement of respiratory parameters may provide a new method for the treatment of CHF. Ipratropium is an inhaled anticholinergic bronchodilator with no reported cardiac or systemic effect. A pilot study was performed to investigate the acute effects of a 72 micrograms inhaled dose of ipratropium bromide on pulmonary function and pulmonary artery pressures in 18 nonsmokers and 11 smokers with severe (New York Heart Association class 2 or 3), stable CHF who were referred for orthotopic cardiac transplantation. An unmatched group of 10 healthy subjects (5 men and 5 women, mean age 36.8 +/- 1.8 years) were studied with pulmonary function testing alone. Forced expiratory volume in 1 second (FEV1) in 15 of 18 nonsmokers with CHF showed a favorable response with a mean improvement of 5.1% (2.74 +/- 0.20 to 2.89 +/- 0.19 liter after drug treatment; p = 0.0026). Forced expiratory flow between 25 and 75% of the forced vital capacity (FEF25-75) improved by 19% (2.50 +/- 0.25 to 3.09 +/- 0.28 liter/s; p = 0.0013). Eight of 11 smokers with CHF responded with a 9.5% increase in FEV1 (2.32 +/- 0.21 to 2.54 +/- 0.19 liter; p = 0.0006) and a 23.2% increase in FEF25-75 (1.82 +/- 0.38 to 2.37 +/- 0.46 liter/s; p = 0.0029). Pulmonary artery pressures, cardiac output, systemic arterial pressures, and cardiac rate and rhythm were unaffected by administration of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of intracellular Ca2+ stores from rat basophilic leukemia cells to study the molecular mechanism leading to quantal Ca2+ release by inositol 1,4,5-trisphosphate

Quantal Ca2+ release is a novel motif for the mediation of signal transduction in which the amplitude of a biological response following multiple stepwise increases in agonist concentration is retained. The release of Ca2+ from permeabilized cells in response to the second messenger inositol 1,4,5-trisphosphate (InsP3) proceeds in this fashion. The mechanisms leading to quantal Ca2+ release are unknown. InsP3 releases 50-90% of the Ca2+ sequestered within the intracellular stores of mammalian cells permeabilized with saponin. However, preparation of microsomes results in the loss of this sensitivity. In this report, functionally intact intracellular Ca2+ stores were isolated from rat basophilic leukemia (RBL) cells by osmotic lysis followed by differential and sucrose density gradient centrifugation. From this preparation, 64% of the stored Ca2+ is released by InsP3. We demonstrate that quantal Ca2+ release is retained by isolated Ca2+ stores and is identical to that observed in permeabilized cells. Addition of a subsaturating (28 nM) concentration of InsP3 to permeabilized cells at 37 degrees C results in the release of only a small fraction of the sequestered Ca2+. When the cells are cooled to 11 degrees C, the remaining Ca2+ is rapidly released. Hence, the mechanism leading to the quantal nature of Ca2+ release is reversible and is thus not likely to be the result of a covalent modification of the channel protein or of the Ca2+ store.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids potentiate contractile response of rabbit myocardium to the beta adrenergic agonist isoproterenol

Opioid agonists and antagonists have both been reported to augment myocardial contractile force in vitro. We reported that the strong opioid agonists morphine and levorphanol, the weak agonist dextrorphan (an optical isomer of levorphanol), and the opioid antagonist naloxone all potentiate the stimulatory effects of the beta-adrenergic agonist isoproterenol on isometric tension generated by isolated rabbit right ventricular myocardium. The EC50 of isoproterenol was found to be shifted leftward 2.7-, 5.4-, 5.3-, and 3.4-fold respectively (p less than 0.05 when compared with controls), when the opioids were added at a final concentration of 1 x 10(5) M. Lower concentrations of opioid or antagonist did not potentiate the effects of isoproterenol. The rank order potency for potentiation thus differs markedly from that of opioid analgesia. The observed potentiation is therefore not agonist specific and not stereospecific. Furthermore, the drugs alone at a range of concentration from 10(-8) to 10(-5) M had no effect on isometric tension generated. We conclude that opioid agonists and antagonists potentiate the response of ventricular myocardium to the effects of beta-adrenergic stimulation by a novel mechanism unrelated to the binding of these drugs to opioid receptors. The paradoxical augmentation of myocardial contractility by either class of agent under a variety of clinical and experimental conditions is thus explained by these findings. Either agent may interact with myocardial tissue to cause increased sensitivity to stimulation by circulating catecholamines.

High-density lipoprotein cholesterol is reduced in patients with sarcoidosis

PURPOSE

Sarcoidosis is a disease in which the proliferation of monocyte-macrophage-derived cells is observed. In other diseases characterized by expansion of the monocyte-macrophage system, such as Gaucher's disease and myeloid metaplasia, abnormalities of lipoprotein metabolism have been demonstrated. To determine whether similar abnormalities in lipoprotein cholesterol concentrations could be identified in patients with sarcoidosis, we studied total cholesterol, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol as well as triglyceride levels in 52 patients with biopsy-proven sarcoidosis.

PATIENTS AND METHODS

Patients had no other medical disorders and were not being treated with corticosteroids or antimalarial agents. Blood samples were collected by venipuncture after an overnight fast. Plasma total cholesterol and triglyceride levels were measured using enzymatic techniques. Lipoprotein cholesterol was quantified by lipoprotein fractionation. HDL cholesterol was measured as cholesterol remaining in the supernatant after precipitation of LDL and very-low-density lipoprotein from whole plasma by the heparin-maganese chloride method. Computation was used to determine the level of LDL cholesterol.

RESULTS

We found significantly reduced levels of total cholesterol (183.9 +/- 27.6 versus 194.3 +/- 16.5 mg/dl, mean +/- SD, p = 0.021) and HDL cholesterol (41.2 +/- 13.0 versus 51.9 +/- 6.1 mg/dl, p = 0.0001) in sarcoid patients versus an age-, sex-, and race-matched reference group. Differences were not observed in triglyceride or LDL cholesterol levels (p greater than 0.05).

CONCLUSION

These findings are similar to those observed in the myeloproliferative diseases, Gaucher's disease, and rheumatoid arthritis and suggest a functional role for monocytes-macrophages in the regulation of serum lipoprotein cholesterol levels.

Lipomatous hypertrophy of the interatrial septum: characterization by transesophageal and transthoracic echocardiography, magnetic resonance imaging, and computed tomography

Lipomatous hypertrophy of the interatrial septum is a benign condition that must be distinguished from other space-occupying lesions of the atria. Patients with this disorder generally have chronic pulmonary disease and thus are difficult to image with conventional transthoracic two-dimensional echocardiography. Transesophageal echocardiography can provide high quality imaging of intracardiac structures in patients who lack adequate transthoracic echocardiographic windows as a result of pulmonary disease. This case report describes the appearance of lipomatous hypertrophy of the interatrial septum as investigated by transesophageal echocardiography.

Modification and inactivation of CoA transferase by 2-nitro-5-(thiocyanato)benzoate

Succinyl-CoA:3-ketoacid coenzyme A transferase undergoes a biphasic reaction with 2-nitro-5-(thiocyanato)-benzoate, giving approximately 70% loss of activity in the initial phase. Active-site titration shows that this inactivation represents the complete loss of activity of 75 +/- 5% of the enzyme molecules. The remaining 25 +/- 5% of the active sites is protected against inactivation by methyl methanethiosulfonate and 5,5'-dithiobis(2-nitrobenzoate); this protection is removed upon treatment of the modified enzyme with dithiothreitol Values of kcat/Km for the two half-reactions catalyzed by the enzyme are the same for the native and modified enzymes on the basis of number of remaining active sites. The modified enzyme shows a smaller decrease in activity with increasing pH in the range pH 7.5-8.7 than the native enzyme. It is concluded that the "essential" thiol group of the enzyme is not involved directly in catalysis and that it reacts with 2-nitro-5-(thiocyanato)-benzoate by two pathways, to form active and inactive enzymes. This can be explained by the attack of the thiol on carbon to form active enzyme-SCN and the attack on sulfur to form enzyme-SSAr, which is blocked at the active site and rapidly undergoes irreversible inactivation.