Phosphatidylcholine and cholesterol inhibit phosphatidate-mediated calcium traversal of liposomal bilayers

Compartmentalization of adenylate cyclase and cAMP signalling

Concepts of cAMP signalling have changed dramatically from the linear cascades of just a few years ago, with the realization that numerous cellular processes affect this motif. These influences include other signalling pathways--most significantly Ca2+, scaffolding proteins (which are themselves variously regulated) to organize the elements of the pathway, and subcellular targeting of components. An obvious implication of this organization is that global measurements of cAMP may trivialize the complexity of the cAMP signals and obscure the regulation of targets. In this presentation, current developments on the targeting and assembly of ACs (adenylate cyclases) and their delivery to selected raft or non-raft domains of the plasma membrane will be discussed, along with the susceptibility of raft-targeted ACs to very discrete modes of increases in the intracellular Ca2+ concentration. Single-cell explorations of cAMP dynamics, as measured with cyclic nucleotide-gated channels, are also described in this paper, particularly as applied to cells in which the composition of AKAP (A-kinase anchoring protein)-PKA (protein kinase A)-PDE (phosphodiesterase) assemblies is probed by RNA interference ablation of defined AKAPs.

Effect of membrane cholesterol on calcium phosphate formation in aqueous suspensions of anionic liposomes

The present study examined the effect of membrane cholesterol on liposome-mediated calcium phosphate precipitation in metastable aqueous solutions (2.25 mM Ca2+ and 1.5 mM inorganic phosphate) at 22 degrees C, pH 7.4 and 240 mOsm. The liposomes were prepared from 7:2:X molar mixtures of phosphatidylcholine, dicetylphosphate, and cholesterol (x = 0, 1, 5, or 9) and contained either 0 or 50 mM encapsulated phosphate. The membranes were made permeable to Ca2+ by addition of the cationophore, X-537A. Changes in external Ca2+ concentration were used as the principal monitor of the course of precipitation. Without encapsulated phosphate, 7:2:X liposomes (with or without ionophore) induced no precipitation. With 50 mM encapsulated phosphate and in the presence of ionophore, precipitation significantly depended on the cholesterol level in the membrane. At 0 and 10 mole% cholesterol, precipitate developed rapidly both within and outside the liposomes. At 35 and 50 mole% cholesterol, no observable intraliposomal precipitation occurred, and extraliposomal precipitation started only after an induction period of 24 hours. Delayed extraliposomal precipitation also took place in PO4-containing liposomes without added ionophore. In this latter case, however, cholesterol was essential for this precipitation to occur with the optimum level being around 10 mole%. Suppression of ionophore-mediated intraliposomal precipitation at higher cholesterol levels could be related to the inflexible cholesterol molecules making the membrane more rigid, thereby restricting Ca-ionophore transport. This restriction could be reversed with ethanol. Delayed extraliposomal precipitation in the absence of added ionophore (or at higher cholesterol levels in its presence) could be explained by seeding from low, unobserved levels of intraliposomal precipitate formed during slow, unfacilitated Ca2+ leakage into the liposomal interior.

Ca2+ interaction with phospholipid bilayers studied by multifrequency phase fluorometry

Calcium interaction with phospholipid membranes containing phosphatidic acid is studied by multifrequency phase fluorometry, using DPH as fluorescent molecule. DPH decay is analysed by a continuous distribution of lifetimes. The results suggest an increase of membrane heterogeneity at low calcium concentrations, without changes in the polarity of the environment surrounding the probe.

Quantitation of Ca2+ uptake by Arsenazo III-loaded liposomes

Arsenazo III-loaded liposomes are in wide use as model systems in the study of Ca2+ transport. The most sophisticated method for quantitation of Ca2+ uptake [E. B. Smaal et al. (1985) Biochim. Biophys. Acta 816, 418] utilizes the absorbance changes (650-700 nm) elicited by sequential additions of EDTA and A23187 to distinguish Ca2+-Arsenazo complexes which are outside and inside the liposomes. In this paper, the analytical approach of Smaal and co-workers is reevaluated and a straightforward treatment that allows calculation both of the concentration of Ca2+ inside liposomes and of total Ca2+ uptake (in moles/mole phospholipid) is developed.

Calcium diphosphatidate membrane traversal is inhibited by common phospholipids and cholesterol but not by plasmalogen

Phosphatidate-mediated Ca2+ membrane traversal is inhibited by phospholipids (PL) such a phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylserine (PS), sphingomyelin and lysoPC, but not by PC-plasmalogen. Kinetics of Ca2+ traversal through a 'passive' bilayer consisting of OH-blocked cholesterol show competition between PC and phosphatidic acid (PA); it appears likely that a Ca(PA.PC) complex is formed which is not a transmembrane ionophore but will reduce the amount of phosphatidic acid available for the formation of the ionophore, Ca(PA)2. PS and PI may inhibit Ca2+-traversal in the same manner by forming Ca(PA.PL) complexes. We suggest that PC-plasmalogen, with one of the Ca2+-chelating ester CO groups missing, cannot engage in calcium cages, i.e., Ca(PA.PL) complexes, and thus does not interfere with Ca(PA)2 formation. Double-reciprocal plotting of Ca2+ traversal rates in cholesterol-containing liposomes vs. calcium concentration suggests that cholesterol inhibits Ca2+ traversal by competing with Ca2+ for PA. The inhibition does not seem to be caused by a restructuring or dehydration of the membrane 'hydrogen belts' affected by cholesterol; most probably, it is due to hydrogen bonding of the cholesterol-OH group to a CO group of PA; this reduces the amount of PA available for the calcium ferry. The inhibition by sphingomyelin and lysoPC may also be explained by their OH group interacting with PA via hydrogen bonding. The pH dependence of Ca2+ traversal suggests that H[Ca(PA)2]- can serve as Ca2+ cross-membrane ferry but that at physiological pH, [Ca(PA)2]2- is the predominant ionophore. In conclusion, the results indicate that Ca2+ traversal is strongly dependent on the structure of the hydrogen belts, i.e., the membrane strata occupied by hydrogen bond acceptors (CO of phospholipids) and donors (OH of cholesterol, sphingosine), and that lipid hydrogen belt structures may regulate storage and passage of Ca2+.

Preparation of passive bilayer liposomes

In studies of in-membrane molecular interactions, need may arise for a matrix that cannot itself interact, except hydrophobically, with the reactants. Such a bilayer matrix should, ideally, consist of only a hydrophobic zone without ionic outer layers and without hydrogen belts (the membrane strata containing CO and OH groups). However, because of the necessity of anchoring the bilayer to its aqueous surroundings, there must be polar substituents. Hydrophilic ether groups in the form of polyoxyethylenes can provide nearly sufficient anchoring and yet not confer unwanted reactivity to the membrane since they are only very weak H-bond acceptors. The stability of the bilayer is ensured by the presence of a few percent of an amphiphile (which may be the substrate to be studied, e.g. a phospholipid) or by a free polyethylene hydroxy group far remote from the original hydrogen belt region. Our most impermeable liposomes consisted of O-methylcholesterol/O-methoxyethoxyethoxyethylcholesterol; the most readily prepared liposomes were made from O-methylcholesterol and hydroxy(ethoxy)4dodecane (Brij 30) or Triton.