Gangliosides modulate sodium transport in cultured toad kidney epithelia

Cultured A6 epithelial cells from toad kidney form confluent monolayers with tight junctions separating the apical and basolateral membranes. These two membrane domains have distinct compositions and functions. Thus, sodium is actively transported across the epithelia from the apical to basolateral surface via amiloride-inhibitable sodium channels located in the apical membrane. Sodium transport is stimulated by vasopressin, cholera toxin, and 8-bromo-cAMP applied to the basolateral surface where the receptors, adenylate cyclase, and Na+/K+-ATPase are located. In a previous study (Spiegel, S., Blumenthal, R., Fishman, P.H., and Handler, J.S. (1985) Biochim. Biophys. Acta 821, 310-318), we demonstrated that exogenous gangliosides inserted into the apical membrane of A6 epithelia do not redistribute to the basolateral membrane. With the ability to vary selectively the ganglioside composition of the apical membrane, we examined the effects of gangliosides on sodium transport in A6 epithelia. When the apical surface of A6 epithelia were exposed to exogenous gangliosides, sodium transport in response to vasopressin, cholera toxin, and 8-bromo-cAMP was enhanced compared to epithelia not exposed to gangliosides. The effect was observed with bovine brain gangliosides, NeuAc alpha 2----3Gal beta 1----3GalNAc beta 1----4[NeuAc alpha 2----3]Gal beta 1----4Glc beta 1----Cer (GD1a) and Gal beta-1----3GalNAc beta 1----4[NeuAc alpha 2----3]Gal beta 1----4Glc beta 1----Cer (GM1), but not with the less complex ganglioside, Neu-Ac alpha 2----3Gal beta 1----4Glc beta 1----Cer (GM3). We examined A6 cells for endogenous gangliosides and found that, whereas GM3 was a major ganglioside, only trace amounts of GM1 and GD1a were present. Based on cell surface and metabolic labeling studies, these gangliosides were synthesized by the cells and were present on the apical as well as the basolateral surface. Bacterial sialidase, which hydrolyzes more complex gangliosides to GM1, was used to modify the endogenous gangliosides on the apical surface; after sialidase treatment, the epithelia were more responsive to vasopressin, cholera toxin, and 8-bromo-cAMP. Thus, gangliosides may be modulators of sodium channels present in the apical membrane of epithelial cells.

Sulfhydryl group(s) in the ligand binding site of the D-1 dopamine receptor: specific protection by agonist and antagonist

An iodinated compound, [125I]-8-iodo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin -7-ol, has been recently reported [Sidhu, A., & Kebabian, J.W. (1985) Eur. J. Pharmacol. 113, 437-440] to be a specific ligand for the D-1 dopamine receptor. Due to its high affinity and specific activity, this ligand was chosen for the biochemical characterization of the D-1 receptor. Alkylation of particulate fractions of rat caudate nucleus by N-ethylmaleimide (NEM) caused an inactivation of the D-1 receptor, as measured by diminished binding of the radioligand to the receptor. The inactivation of the receptor sites by NEM was rapid and irreversible, resulting in a 70% net loss of binding sites. On the basis of Scatchard analysis of binding to NEM-treated tissue, the loss in binding sites was due to a net decrease in the receptor number with a 2-fold decrease in the affinity of the receptor for the radioligand. Receptor occupancy by either a D-1 specific agonist or antagonist protected the ligand binding sites from NEM-mediated inactivation. NEM treatment of the receptor in the absence or presence of protective compound abolished the agonist high-affinity state of the receptor as well as membrane adenylate cyclase activity. The above-treated striatal membranes were fused with HeLa membranes and assayed for dopamine-stimulated adenylate cyclase activity. When the sources of D-1 receptors were from agonist-protected membranes, the receptors retained their ability to functionally couple to the HeLa adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of the beta-adrenergic receptor-coupled adenylate cyclase in cultured mammalian cells. Receptor sequestration versus receptor function

Human A431 and rat glioma C6 cells exposed to isoproterenol underwent a time- and dose-dependent loss of isoproterenol-stimulated adenylate cyclase activity. Desensitization was accompanied by sequestration of beta-adrenergic receptors, which became less accessible to the hydrophilic antagonist 3H-labeled 4-(3-tert-butylamino-2-hydroxypropoxy)benzimidazole-2-one hydrochloride ([3H]CGP-12177) and redistributed from the heavier density plasma membrane fraction to a lighter density membrane fraction. Prior treatment of the cells with concanavalin A or phenylarsine oxide blocked sequestration of the receptors but not desensitization of the agonist-stimulated adenylate cyclase. The membranes from such pretreated cells were exposed to alkali to inactivate adenylate cyclase, and the receptors were transferred to a foreign adenylate cyclase by membrane fusion with polyethylene glycol. beta receptors from desensitized cells exhibited a reduced ability to maximally stimulate the foreign adenylate cyclase, but remained accessible to [3H]CGP-12177 in the fused membranes. When isoproterenol-treated cells were washed free of agonist, there was a time-dependent recovery of agonist responsiveness and [3H]CGP-12177-binding sites. Using the fusion technique, the receptors recovered their functional activity in the resensitized cells. In concanavalin A-treated cells, desensitization and resensitization appeared to occur in the absence of receptor sequestration. Finally, membranes from desensitized cells pretreated with concanavalin A were fused with polyethylene glycol and assayed for agonist-stimulated adenylate cyclase. There was no reversal of the desensitized state. Thus, the primary, essential step in the desensitization process is a reduction in functional activity of the beta-adrenergic receptor. In contrast, sequestration of the receptors is not a prerequisite, but a secondary event during desensitization.

Reevaluation of the role of gangliosides as receptors for tetanus toxin

Binding of tetanus toxin to rat brain membranes was of lower affinity and capacity when binding was determined in 150 mM NaCl, 50 mM Tris-HCl (pH 7.4) than in 25 mM Tris-acetate (pH 6.0). Binding under both conditions was reduced by treating the membranes with neuraminidase. Pronase treatment, however, reduced toxin binding only in the Tris-saline buffer (pH 7.4). In addition, the concentration of gangliosides required to inhibit toxin binding was 100-fold higher in Tris-saline compared to Tris-acetate buffer. The toxin receptors in the membranes were analyzed by ligand blotting techniques. Membrane components were dissolved in sodium dodecyl sulfate, separated by polyacrylamide gel electrophoresis, and transferred to nitrocellulose sheets, which were overlaid with 125I-labeled toxin. Tetanus toxin bound only to material that migrated in the region of the dye front and was extracted with lipid solvents. Gangliosides isolated from the lipid extracts or other sources were separated by TLC on silica gel and the chromatograms were overlaid with labeled tetanus toxin. The toxin bound to areas where the major rat brain gangliosides migrated. When equimolar amounts of different purified gangliosides were applied to the chromatogram, binding of the toxin was in the order GD1b approximately equal to GT1b approximately equal to GQ1b greater than GD2 greater than GD3 much greater than GD1a approximately equal to GM1. Thus, the toxin appears to have the highest affinity for gangliosides with a disialyl group linked to the inner galactosyl residue. When binding of tetanus toxin to transfers and chromatograms was determined in the Tris-saline buffer (pH 7.4), the toxin bound to the same components but the extent of binding was markedly reduced compared with the low-salt and -pH conditions. Our results indicate that the interaction of tetanus toxin with rat brain membranes and gangliosides is greatly reduced under more physiological conditions of salt and pH and raise the possibility that other membrane components such as sialoglycoproteins may be receptors for the toxin under these conditions.

Intravitreal liposome-encapsulated gentamicin in a rabbit model. Prolonged therapeutic levels

The authors investigated the effect of liposome encapsulation on the pharmacokinetics of gentamicin after intravitreal injection in albino rabbits, using immunofluorescent assay. Gentamicin was encapsulated into liposomes of phosphatidylcholine, phosphatidic acid, and alpha-tocopherol. The final liposomal suspension contained gentamicin, 10 mg/ml, 95% encapsulated. One eye of each rabbit received an intravitreal injection (100 mg gentamicin per 0.1 ml) of either liposome-encapsulated gentamicin (LEG) or gentamicin in phosphate-buffered saline (Gs). Equilibrium dialysis separated free from encapsulated drug in vitreous samples. The peak free drug concentration was significantly less (P less than .01, unpaired t-test) with LEG than with Gs. Concentrations of free and total gentamicin were significantly greater (P less than .05) with LEG than with Gs at 24, 72, 120, and 192 hr. LEG gave a twofold increase in area under the drug concentration time curve for total drug, and a 1.5-fold increase for free drug when compared to Gs.

Solubilization of the D-1 dopamine receptor from rat striatum

The D-1 dopamine receptor was extracted from rat striatal membranes with 0.7% sodium cholate and 1 M NaCl. Pretreatment of the membranes with a D-1 specific agonist, inclusion of crude phospholipids in the solubilization buffer, and subsequent removal of the detergent led to a maximal extraction of 48% of the receptor binding sites. The D-1 antagonist, [125I]SCH 23982, bound to single class of sites with a Kd of 1.8 nM and a Bmax of 1.65 pmol/mg protein. The solubilized receptors retained the ability to discriminate between active and inactive enantiomers of agonists and antagonists selective for the D-1 receptor.

Gonadotropin-mediated desensitization in a murine Leydig tumor cell line does not alter the regulatory and catalytic components of adenylate cyclase

MLTC-1 cells, derived from a murine Leydig tumor, contain a gonadotropin-responsive adenylate cyclase that became desensitized to hCG. Prior exposure to hCG reduced the ability of MLTC-1 cells to accumulate cAMP by approximately 50%, but caused only a small reduction in hCG receptor number. Membranes isolated from desensitized cells showed a similar reduction in hCG-stimulated adenylate cyclase activity. Desensitization was time, temperature, and dose dependent. Elevating intracellular cAMP levels by incubating the cells with (Bu)2cAMP or cholera toxin failed to cause desensitization. Desensitization did not depend on protein synthesis. Desensitization caused no change in the dose response of adenylate cyclase to hCG or GTP. hCG receptor affinity for hCG was not affected by desensitization or guanine nucleotides. The stimulatory regulatory component of adenylate cyclase (Ns) from MLTC-1 cells was used to reconstitute S49 cyc- membranes, which lack Ns. Ns from control and desensitized MLTC-1 cells were equally effective in reconstitution of the beta-adrenergic-sensitive adenylate cyclase of cyc-. beta-Adrenergic receptors from cyc- membranes were also transferred to MLTC-1 membranes by fusion with polyethylene glycol to produce a beta-adrenergic-responsive adenylate cyclase. Isoproterenol-stimulated activity was similar, regardless of whether membranes from control or desensitized MLTC-1 cells were used. We conclude that neither Ns nor the catalytic subunit of the adenylate cyclase in MLTC-1 cells is the site of lesion in desensitization. Most likely, the hCG receptor itself may be affected when MLTC-1 cells are desensitized by hCG.

A preliminary study of corneal penetration of 125I-labelled idoxuridine liposome

We compared corneal penetration of idoxuridine (IDU) to liposome-encapsulated IDU. Liposomes of phosphatidic acid, phosphatidyl choline, and alpha-tocopherol in a molar ratio 1:8:1 were prepared using the reverse phase evaporation method. New Zealand albino rabbits received either 0.1% solution of I125-labelled aqueous IDU or liposome-encapsulated IDU topically every 2 min for 6 min. Corneal, aqueous, and vitreous samples were assayed for I125 radioactivity at 15 min and at 1, 2, 3, and 6 h following drug application. Our results indicated that corneal penetration of liposomal IDU was significantly increased over the regular form of the drug for a time interval of 6 h.

Fibrillar organization of fibronectin is expressed coordinately with cell surface gangliosides in a variant murine fibroblast

NCTC 2071A cells, a line of transformed murine fibroblasts, grow in serum-free medium, are deficient in gangliosides, synthesize fibronectin, but do not retain and organize it on the cell surface. When the cells are exposed to exogenous gangliosides, fibrillar strands of fibronectin become attached to the cell surface. A morphologically distinct variant of NCTC 2071A cells was observed to both retain cell surface fibronectin and organize it into a fibrillar network when the cells were stained with anti-fibronectin antibodies and a fluorescent second antibody. A revertant cell type appeared to resemble the parental NCTC 2071A cells in terms of morphology and fibronectin organization. All three cell types were subjected to mild NaIO4 oxidation and reduction with KB3H4 of very high specific radioactivity in order to label the sialic acid residues of surface gangliosides. The variant had much more surface gangliosides than the parental, particularly more complex gangliosides corresponding to GM1 and GD1a. The surface gangliosides of the revertant were intermediate between the parental and the variant. By using sialidase, which hydrolyzes GD1a to GM1, and 125I-labeled cholera toxin, which binds specifically to GM1, the identity and levels of these gangliosides were confirmed in the three cell types. When variant cells were exposed to sialidase for 2 d, there appeared to be little change in fibronectin organization. Concomitant treatment of the cells with the B subunit of cholera toxin, which bound to all the surface GM1 including that generated by the sialidase, however, eliminated the fibrillar network of fibronectin. In addition, exposure of the variant cells to a 70,000-mol-wt fragment of fibronectin, which lacks the cell attachment domain but contains a matrix assembly domain, inhibited the formation of fibers. Finally, all three cell types were assayed for their ability to attach to and spread on fibronectin-coated surfaces; no significant differences were found. Our results further establish that the ability of a cell to organize fibronectin into an extracellular matrix is dependent on certain gangliosides, but they also indicate that cell adhesion to fibronectin is independent of these gangliosides. We suggest that matrix organization and cell attachment and spreading are based on separate mechanisms and that these functions are associated with different cell surface "receptors."

The effect of ultraviolet, visible and infrared radiation on the rabbit blood-aqueous barrier

We studied the effects of visible (400-750 nm), infrared (750-1100 nm), and ultraviolet (300-400 nm) radiation on the breakdown of the blood-aqueous barrier (BAB) in albino rabbit eyes using fluorophotometry and fluorescein-bound albumin. There was no evidence of BAB breakdown following exposure to visible (158 J cm-2) or infrared (106 J cm-2) radiation. Fluorescence was detected in the anterior chamber 15 min following ultraviolet (UV) (300-400 nm; 37 J cm-2 and 73 J cm-2) and ultraviolet-A (UV-A) (320-400 nm; 45 J cm-2 and 90 J cm-2) irradiation. This fluorescence peaked at 2 hr after irradiation. Reestablishment of the BAB, indicated by no detectable fluorescence, occurred in all animals after 24 hr. There was a significant increase in fluorescence with increasing UV-irradiation dosage in all groups (P less than 0.05). Comparison of BAB breakdown following exposure to ultraviolet 320-400 nm and 300-400 nm in the same dosage (45 J cm-2) indicated that the inclusion of shorter wavelength UV exposure produced a significantly higher degree of BAB breakdown (P less than 0.001).

Direct evidence that endogenous GM1 ganglioside can mediate thymocyte proliferation

The B subunit of cholera toxin, which is multivalent and binds exclusively to a specific ganglioside, GM1, was mitogenic for rat thymocytes. When exposed to the B subunit, the cells proliferated, as measured by 3H-labeled thymidine incorporation. Mitogenesis depended on the direct interaction of the B subunit with GM1 on the surface of the cells. This demonstrates that endogenous plasma membrane gangliosides can mediate proliferation in lymphocytes.

Gangliosides do not move from apical to basolateral plasma membrane in cultured epithelial cells

Both qualitative and quantitative approaches were used to ascertain whether gangliosides, incorporated into the apical plasma membrane of cultured epithelial cells from kidney of toad (A6) and dog (MDCK), were able to redistribute past the tight junctions to the basolateral membrane. The apical surfaces of confluent epithelia were exposed to rhodaminyl gangliosides and the distribution of the inserted gangliosides was assessed qualitatively by fluorescence microscopy. All of the fluorescence was confined to the apical surface for at least 1 h after the fluorescent gangliosides had become incorporated; none appeared on the basolateral surface. These observations were confirmed by incubating the cells with anti-rhodamine antibodies and 125I-labeled protein A. In order to quantitate further the ganglioside distribution, binding assays were performed using 125I-labeled cholera toxin, which binds specifically to ganglioside GM1. Exogenous GM1 added to the apical membrane was not detected on the basolateral membrane 4 h after its incorporation even though there was extensive disappearance of the inserted ganglioside, presumably through endocytosis. To directly examine the behaviour of endogenous gangliosides, the apical surface of the epithelial cells was exposed to bacterial neuraminidase, which hydrolyzes more complex gangliosides to GM1. The cells exhibited a 10-fold increase in binding of cholera toxin to their apical surface, but no increase in binding to their basolateral surface. Thus, no cellular pathways for movement from apical to basolateral plasma membrane appear to be available for implanted or endogenous gangliosides.

Monoclonal IgM in a patient with paraproteinemic polyneuropathy binds to gangliosides containing disialosyl groups

Monoclonal IgM kappa from a patient with polyneuropathy associated with paraproteinemia was found to bind to several polysialogangliosides. Binding of IgM to gangliosides was shown by enzyme-linked immunosorbent assay (ELISA) and by overlaying thin-layer chromatograms of human brain and peripheral nerve gangliosides with the patient's serum followed by radioiodinated goat antihuman IgM. The latter technique showed that the IgM paraprotein reacted with a number of gangliosides. In an ELISA the IgM paraprotein reacted strongly with GD2, GD3, GD1b, and GT1b, but not with GM1, GM3, and GD1a. Thus, the epitope for the patient's IgM paraprotein appears to involve the disialosyl configurations.

Phorbol esters and beta-adrenergic agonists mediate desensitization of adenylate cyclase in rat glioma C6 cells by distinct mechanisms

Exposure of rat glioma C6 cells to either isoproterenol or 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in desensitization of isoproterenol-stimulated adenylate cyclase activity. After either treatment, the affinity of beta-receptors for isoproterenol was reduced. Thus, desensitization by TPA or isoproterenol appeared to involve an "uncoupling" of the beta-receptor from the stimulatory regulatory component (Ns) of adenylate cyclase. The activity of Ns, assayed by reconstitution of S49 cyc- adenylate cyclase activity, was found to be unchanged after desensitization. The activity of beta-receptors was measured by inactivating Ns and the catalytic component of adenylate cyclase in C6 membranes and fusing them with membranes lacking beta-receptors. Receptors from isoproterenol-treated C6 cells were less active in "coupling" to the foreign adenylate cyclase than receptors from untreated cells, whereas receptors from TPA-treated cells were fully active. This unexpected latter result was explored further. Lysates from C6 cells were centrifuged on linear sucrose density gradients and the gradient fractions assayed for beta-receptor binding activity. Most of the receptors were recovered in a "heavy" plasma membrane peak but some receptors also appeared in a "light" membrane peak. After treatment of the cells with isoproterenol or TPA, the proportion of receptors in the light peak increased. Prior treatment of the cells with concanavalin A prevented the increase in light receptors caused by isoproterenol or TPA. In addition, the concanavalin A treatment prevented the desensitization of adenylate cyclase caused by TPA but not that caused by isoproterenol. Finally, desensitization of adenylate cyclase was reversed by polyethylene glycol-induced fusion of membranes from cells treated with TPA but not isoproterenol. We conclude that beta-agonists and phorbol esters desensitize adenylate cyclase by distinct mechanisms. Agonists cause a reduction in the functional activity of the beta-receptors followed by a segregation of the receptors into a light membrane fraction devoid of Ns. Phorbol esters do not alter the activity of the receptors but do cause their segregation.

Alterations in the blood-aqueous barrier of the rabbit eye after neodymium: YAG photodisruption

We studied the effects of the Nd:YAG laser on the blood-aqueous barrier following photodisruption of the anterior capsule, posterior capsule, and mid-vitreous of the albino rabbit eye with fluorophotometric techniques using albumin-bound fluorescein. After photo-disruption of the anterior capsule, fluorescence appeared in the anterior chamber at 30 minutes, peaked at two hours, and decreased almost to baseline values by 24 hours. No fluorescence was noted at any time in contralateral control eyes or in eyes receiving photo-disruption of the posterior capsule or mid-vitreous.

Fate of tetanus toxin bound to the surface of primary neurons in culture: evidence for rapid internalization

We examined the nature of the tetanus toxin receptor in primary cultures of mouse spinal cord by ligand blotting techniques. Membrane components were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose sheets, which were overlaid with 125I-labeled tetanus toxin. The toxin bound only to material at or near the dye front, which was lost when the cells were delipidated before electrophoresis. Gangliosides purified from the lipid extract were separated by thin-layer chromatography and the chromatogram was overlaid with 125I-toxin. The toxin bound to gangliosides corresponding to GD1b and GT1b. Similar results were obtained with brain membranes; thus, gangliosides rather than glycoproteins appear to be the toxin receptors both in vivo and in neuronal cell cultures. To follow the fate of tetanus toxin bound to cultured neurons, we developed an assay to measure cell-surface and internalized toxin. Cells were incubated with tetanus toxin at 0 degree C, washed, and sequentially exposed to antitoxin and 125I-labeled protein A. Using this assay, we found that much of the toxin initially bound to cell surface disappeared rapidly when the temperature was raised to 37 degrees C but not when the cells were kept at 0 degree C. Some of the toxin was internalized and could only be detected by our treating the cells with Triton X-100 before adding anti-toxin. Experiments with 125I-tetanus toxin showed that a substantial amount of the toxin bound at 0 degree C dissociated into the medium upon warming of the cells. Using immunofluorescence, we confirmed that some of the bound toxin was internalized within 15 min and accumulated in discrete structures. These structures did not appear to be lysosomes, as the cell-associated toxin had a long half-life and 90% of the radioactivity released into the medium was precipitated by trichloroacetic acid. The rapid internalization of tetanus toxin into a subcellular compartment where it escapes degradation may be important for its mechanism of action.

Fluorescent gangliosides as probes for the retention and organization of fibronectin by ganglioside-deficient mouse cells

Ganglioside-deficient transformed mouse fibroblasts (NCTC 2071A cells), which grow in serum-free medium, synthesize fibronectin but do not retain it on the cell surface. When fluorescent derivatives of gangliosides, containing either rhodamine or Lucifer yellow CH attached to the sialic acid residues, were added to the culture medium, the cells incorporated the derivatives and their surfaces became highly fluorescent. When the cells were stained with anti-fibronectin antibodies and a fluorescent second antibody, fibrillar strands of fibronectin were observed to be attached to the cell surface, with partial coincidence of the patterns of direct ganglioside fluorescence and indirect fibronectin immunofluorescence at the cell surface. When the cells were exposed to bacterial neuraminidase during the time of ganglioside insertion, similar patterns of fluorescence were observed. Because the fluorescent gangliosides are resistant to the enzyme, these results suggest that neuraminidase-sensitive endogenous glycoconjugates were not involved in the ganglioside-mediated retention and organization of endogenous fibronectin. After cells were exposed to exogenous chicken fibronectin, most of the fibronectin was attached to the substratum and only a few fibrils were attached to the cells. When exogenous gangliosides were included in the incubation, there was a striking increase in cell-associated exogenous fibronectin, which was highly organized into a fibrillar network. Conversely, cells incubated for 18 h with exogenous unmodified gangliosides exhibited a highly organized network of endogenously derived fibronectin. Upon further incubation of the cells for 2 h with fluorescent gangliosides, there was considerable co-distribution of the fluorescent gangliosides with the fibronectin network as revealed by immunofluorescence. Our results support the concept that gangliosides can mediate the attachment of fibronectin to the cell surface and its organization into a fibrillar network.

Effect of inoculum size on the induction of endophthalmitis in aphakic rabbit eyes

A reproducible animal model is necessary to examine the use of antimicrobial agents for prophylaxis and treatment of bacterial endophthalmitis. We determined the minimum inoculum size of S. aureus and P. aeruginosa that consistently produced endophthalmitis when injected into aphakic rabbit eyes immediately following surgery. Both anterior chamber and intravitreal injections were examined. For S. aureus, an intravitreal inoculum of 19.3 +/- 7.5 CFU and an anterior chamber inoculum of 50.5 +/- 4.0 CFU were required. For P. aeruginosa, an intravitreal inoculum of 5.5 +/- 2.6 CFU and an anterior chamber inoculum of 97.5 +/- 10.7 CFU consistently produced a fulminant infection. Lower inocula of both bacteria produced endophthalmitis in both locations, but the effect was inconsistent.

Down-regulation of gonadotropin and beta-adrenergic receptors by hormones and cyclic AMP

Loss of gonadotropin receptors in murine Leydig tumor cells and of beta-adrenergic receptors in rat glioma C6 cells occurred following exposure of the cells to human chorionic gonadotropin and isoproterenol, respectively. Down-regulation of receptors was mimicked in part by other agents that elevated cyclic AMP levels in the cells such as cholera toxin and dibutyryl cyclic AMP. Whereas agonist-mediated receptor loss was rapid and almost total, down-regulation by cyclic AMP was slower and less extensive. Down-regulation of receptors did not appear to be accompanied by loss of the regulatory and catalytic components of adenylate cyclase. Hormone-mediated down-regulation was preceded by desensitization of hormone-stimulated adenylate cyclase. In contrast, there was no evidence that cyclic AMP caused desensitization. Finally, loss of receptors induced either by agonists or cyclic AMP required protein synthesis as cycloheximide inhibited down-regulation. We conclude that down-regulation of receptors in these cells is a complex process involving both cyclic AMP-independent and -dependent events.

A sensitive enzymatic assay for determination of cholesterol in lipid extracts

A procedure for the determination of free and total cholesterol in lipid extracts is described. The method for free cholesterol employs cholesterol oxidase to generate H2O2 and peroxidase to catalyze the reaction of H2O2 with o-dianisidine to yield a colored product. For the determination of total cholesterol, cholesterol ester hydrolase is included.

Functional alteration of the beta-adrenergic receptor during desensitization of mammalian adenylate cyclase by beta-agonists

Exposure of several mammalian cell lines to isoproterenol resulted in a desensitization of the beta-adrenergic receptor-adenylate cyclase system in membranes isolated from the cells. Under the experimental conditions chosen, desensitization was accompanied by a minimal loss of beta-receptors. The cells tested included HeLa, S49 cyc- lymphoma, and rat glioma C6. The functional activity of the beta-receptors was determined by coupling them to a foreign adenylate cyclase by membrane fusion. The donor membranes were treated to inactivate the regulatory and catalytic components of adenylate cyclase. The acceptor membranes were from Friend erythroleukemic cells (Fr cells), which lack beta-receptors, and HeLa cells treated overnight with isoproterenol to eliminate their receptors. The fused membranes were assayed for agonist-stimulated activity, which was always reduced when the donor beta-receptors were from the desensitized membranes. The desensitization appeared to be specific for beta-receptors, as the activity of other receptors and cyclase components was not altered. By fusing HeLa membranes with intact Fr cells, we directly measure the intrinsic activity of native and desensitized beta-receptors. For an equal amount of transferred beta-receptors, the activity was 40%-50% lower when the donor membranes were from desensitized cells. Our results clearly indicate that desensitization mediated by a beta-agonist in mammalian cells results in a functional alteration of the beta-receptor.

Direct visualization of redistribution and capping of fluorescent gangliosides on lymphocytes

Fluorescent derivatives of gangliosides were prepared by oxidizing the sialyl residues to aldehydes and reacting them with fluorescent hydrazides. When rhodaminyl gangliosides were incubated with lymphocytes, the cells incorporated them in a time- and temperature-dependent manner. Initially, the gangliosides were evenly distributed on the cell surface but were redistributed into patches and caps by antirhodamine antibodies. When the cells were then stained with a second antibody or protein A labeled with fluorescein, the fluorescein stain revealed the coincident movement of both the gangliosides and the antirhodamine antibodies. When the cells were treated with both rhodamine and Lucifer yellow CH-labeled gangliosides, the antirhodamine antibodies induced patching and capping of both fluorescent gangliosides but had no effect on cells incubated only with Lucifer yellow CH-labeled gangliosides. In addition, capping was observed on cells exposed to cholera toxin, antitoxin antibodies, and rhodamine-labeled protein A, indirectly showing the redistribution of endogenous ganglioside GM1, the cholera toxin receptor. By incorporating Lucifer yellow CH-labeled GM1 into the cells and inducing capping as above, we were able to demonstrate directly the coordinate redistribution of the fluorescent GM1 and the toxin. When the lymphocytes were stained first with Lucifer yellow CH-labeled exogenous ganglioside GM3, which is not a toxin receptor, there was co-capping of endogenous GM1 (rhodamine) and exogenous GM3 (Lucifer yellow CH). These results suggest that gangliosides may self-associate in the plasma membrane which may explain the basis for ganglioside redistribution and capping.

Desensitization of catecholamine-stimulated adenylate cyclase and down-regulation of beta-adrenergic receptors in rat glioma C6 cells. Role of cyclic AMP and protein synthesis

When exposed to the beta-agonist (-)-isoproterenol, rat glioma C6 cells exhibited a time-and concentration-dependent reduction in isoproterenol responsiveness (desensitization) and a loss of beta-adrenergic receptors (down-regulation). Other agents, such as dibutyryl cyclic AMP, isobutylmethylxanthine, and cholera toxin, all of which elevate intracellular cyclic AMP levels, also induced receptor down-regulation but at a much slower rate than isoproterenol. Loss of beta-receptors was detected with intact cells, cell lysates, and cell membranes. Receptor loss was accompanied by a reduction in isoproterenol-stimulated cyclic AMP production and adenylate cyclase activity. For a given amount of receptor loss, this reduction was much greater with isoproterenol than with other agents. In addition, the concentration of isoproterenol required for half-maximal stimulation of cyclic AMP production was increased in cells treated with isoproterenol but not with isobutylmethylxanthine or dibutyryl cyclic AMP. The affinity of beta-receptors for the agonist was also lower in membranes from cells treated with isoproterenol but not the other agents. Prior treatment of the cells with cycloheximide inhibited receptor loss by isoproterenol but did not prevent desensitization or reduced affinity of beta-receptors for the agonist. Cycloheximide also blocked the loss of receptors induced by dibutyryl cyclic AMP and, in addition, prevented a reduction in agonist-stimulated adenylate cyclase activity. We propose that desensitization is mediated in rat glioma C6 cells only by agonists and is not dependent on either cyclic AMP or protein synthesis. Down-regulation can be induced both by agonists and by cyclic AMP and does depend on protein synthesis. Thus, desensitization and down-regulation can occur independently.

Incorporation of fluorescent gangliosides into human fibroblasts: mobility, fate, and interaction with fibronectin

Rhodamine- and fluorescein-labeled gangliosides were used as probes to investigate the distribution, dynamics, and fate of plasma membrane-bound gangliosides on cultured human fibroblasts. When sparse cultures of fibroblasts were incubated with the fluorescent ganglioside derivatives, their surfaces became highly fluorescent. The fluorescent gangliosides were taken up by the cells in a time- and temperature-dependent manner and were not removed from the cell surface by trypsin or serum. Thus, the gangliosides appeared to be stably incorporated into the lipid bilayer of the plasma membrane. Fluorescent photobleaching recovery measurements showed that the inserted gangliosides were free to diffuse in the plane of the membrane with a high diffusion coefficient of approximately 10(-8) cm2/s. When the ganglioside-treated cells were washed and incubated in fresh medium, the surface gangliosides became internalized with time, and localized in the perinuclear region of the fibroblasts. In dense cultures of fibroblasts, a large fraction of the fluorescent gangliosides were organized in a fibrillar network and were immobile on the time scale of fluorescent photobleaching recovery measurements. Using antifibronectin antibodies and indirect immunofluorescence, these gangliosides were found to co-distribute with fibrillar fibronectin. Thus, exogenous gangliosides appear to be stably inserted into the lipid bilayer of the plasma membrane and to diffuse freely in its plane as well as form a less mobile state with the fibrillar networks of fibronectin associated with the cells.

Antibodies against human chorionic gonadotropin convert the deglycosylated hormone from an antagonist to an agonist

Chemical deglycosylation of human chorionic gonadotropin (hCG) produced an antagonist (DG-hCG) that specifically bound to hCG receptors but was no longer able to stimulate adenylate cyclase in the murine Leydig tumor cell line, MLTC-1. DG-hCG was restored to an agonist by incubating cells or membranes having the bound analogue with antibodies against hCG (anti-hCG). In the presence of anti-hCG, cyclic AMP accumulation and adenylate cyclase activity were stimulated over DG-hCG alone. There was no accumulation of cyclic AMP when the cells were exposed to anti-hCG alone or DG-hCG and normal serum or anti-hCG first then DG-hCG. Several different batches of anti-hCG were effective but their activity did not correlate with their affinity for DG-hCG or hCG. The effect of anti-hCG on DG-hCG activity was dose- and time-dependent. Maximal stimulation of cyclic AMP was achieved with antisera dilutions of 1:200 or less. When DG-hCG-treated cells were exposed to anti-hCG at 37 degrees C, there was a 10-min lag. The lag was eliminated when the cells were exposed to the antibodies at 4 degrees C for 3 h and then warmed to 37 degrees C. Adenylate cyclase was also activated when Fab fragments prepared by papain digestion of anti-hCG were used, whereas Fc fragments were not effective. Thus, the divalency of the anti-hCG is not the critical factor in the mechanism of antibody action. Our results suggest that anti-hCG converts DG-hCG from an antagonist to an agonist possibly by altering the conformation of the modified hormone.