Unusual low proton permeability of liposomes prepared from the endogenous myelin lipids

A Stable Irinotecan Liposome with Enhanced Antitumor Activity in a Range of Tumor Models

PURPOSE

This study aimed to prepare a stable irinotecan liposome (CPT-11 liposome) and evaluate its antitumor efficacy in a range of tumor models.

METHODS

CPT-11 liposome was prepared with a Z-average particle size of 110 ~ 120 nm and high entrapment efficiency (> 95%) and had a good stability within 18 months. Then the antitumor efficacy was studied in human colon (Ls-174t), gastric (NCI-N87), pancreatic (BxPC-3) and small cell lung (NCI-H526) cancer xenograft models. The toxicity of high-dose CPT-11 liposome was also evaluated in Beagle dogs.

RESULTS

The results showed that the anti-tumor effects of CPT-11 liposome were markedly superior (at least 10 times higher) to those of the CPT-11 injection group in all four xenograft models. The tissue distribution test in the Ls-174t model further demonstrated that the CPT-11 liposome could alter the plasma and tissue distribution of CPT-11, increase the exposure level of its active metabolite SN-38 in tumor, and ultimately improve antitumor efficiency. Meanwhile, CPT-11 liposome showed a much less toxicity than CPT-11 injection in beagle dogs.

CONCLUSIONS

Overall, the CPT-11 liposome may be developed as a new clinical alternative for the cancer patients.

Protonic conductor: better understanding neural resting and action potential

With the employment of the transmembrane electrostatic proton localization theory with a new membrane potential equation, neural resting and action potential is now much better understood as the voltage contributed by the localized protons/cations at a neural liquid- membrane interface. Accordingly, the neural resting/action potential is essentially a protonic/cationic membrane capacitor behavior. It is now understood with a newly formulated action potential equation: when action potential is <0 (negative number), the localized protons/cations charge density at the liquid-membrane interface along the periplasmic side is >0 (positive number); when the action potential is >0, the concentration of the localized protons and localized nonproton cations is <0, indicating a "depolarization" state. The nonlinear curve of the localized protons/cations charge density in the real-time domain of an action potential spike appears as an inverse mirror image to the action potential. The newly formulated action potential equation provides biophysical insights for neuron electrophysiology, which may represent a complementary development to the classic Goldman-Hodgkin-Katz equation. With the use of the action potential equation, the biological significance of axon myelination is now also elucidated as to provide protonic insulation and prevent any ions both inside and outside of the neuron from interfering with the action potential signal, so that the action potential can quickly propagate along the axon with minimal (e.g., 40 times less) energy requirement.NEW & NOTEWORTHY The newly formulated action potential equation provides biophysical insights for neuron electrophysiology, which may represent a complementary development to the classic Goldman-Hodgkin-Katz equation. The nonlinear curve of the localized protons/cations charge density in the real-time domain of an action potential spike appears as an inverse mirror image to the action potential. The biological significance of axon myelination is now elucidated as to provide protonic insulation and prevent any ions from interfering with action potential signal.

Hypothesis of an energetic function for myelin

Nervous system is a great oxygen consumer, but the site of oxygen absorption has remained elusive. Four proteomic studies have shown that the respiratory complexes I to V may be expressed in isolated myelin. Myelin is an outgrowth of glial cells, surrounding many axons in multiple spires both in peripheral and central nervous system. Recent quantitative analyses strongly support the daring hypothesis that myelin is functional in aerobic ATP production, to supply the neuron with chemical energy. A vision of myelin sheath as a structure devoted to the oxygen absorbance for glucose combustion in nervous system thank to its enormous surface, would be also supported by an impressive series of characteristics and properties of myelin that do not presently find an explanation, all of which are herein examined.

Evidence for aerobic ATP synthesis in isolated myelin vesicles

Even though brain represents only 2-3% of the body weight, it consumes 20% of total body oxygen, and 25% of total body glucose. This sounds surprising, in that mitochondrial density in brain is low, while mitochondria are thought to be the sole site of aerobic energy supply. These data would suggest that structures other than mitochondria are involved in aerobic ATP production. Considering that a sustained aerobic metabolism needs a great surface extension and that the oxygen solubility is higher in neutral lipids, we have focused our attention on myelin sheath, the multilayered membrane produced by oligodendrocytes, hypothesizing it to be an ATP production site. Myelin has long been supposed to augment the speed of conduction, however, there is growing evidence that it exerts an as yet unexplained neuro-trophic role. In this work, by biochemical assays, Western Blot analysis, confocal laser microscopy, we present evidence that isolated myelin vesicles (IMV) are able to consume O(2) and produce ATP through the operation of a proton gradient across their membranes. Living optic nerve sections were exposed to MitoTracker, a classical mitochondrial dye, by a technique that we have developed and it was found that structures closely resembling nerve axons were stained. By immunohistochemistry we show that ATP synthase and myelin basic protein colocalize on both IMV and optic nerves. The complex of data suggests that myelin sheath may be the site of oxygen absorption and aerobic metabolism for the axons.

Tautomerism of acridin-9-amines substituted at the exocyclic nitrogen atom: spectroscopic investigations and theoretical studies

In the ground electronic state, the first two (1 and 2) of the compounds investigated--9-(methoxyamino)acridine (1), 9-hydrazinoacridine (2), N-(2-chloroethyl)acridin-9-amine (3) and N-(5-methylpyridin-2-yl)acridin-9-amine (4)--exist principally in the imino tautomeric form, while the other two (3 and 4) can coexist as amino and imino tautomers in solvents of various polarities and abilities to enter into specific interactions. These features of the molecules are reflected in the experimental absorption spectra and the predicted thermodynamic and spectral data. The predicted thermodynamic characteristics suggest that in the S1 state, the imino tautomers of 1, 2 and 4 and the amino tautomer of 3 are more stable than their tautomeric counterparts. However, the predicted rates of intersystem crossing suggest that the imino tautomers of 1-3 and the amino tautomer of 4 lose excitation energy very rapidly, so that only their counterpart tautomers in fact emit radiation. This explains why 1 and 2 do not fluoresce and why the amino form of 3 and the imino form of 4 are the emitters. 3 and 4 thus represent acridin-9-amines whose imino forms are preferred in the ground state, but whose respective amino and imino forms are preferred in the excited state. Because 3 and 4 are capable of tautomeric transformations in the ground and excited states, and also enter into specific interactions with solvents, these compounds could be potent spectral indicators or probes of environmental properties.

Polymorphic phases of galactocerebrosides: spectroscopic evidence of lamellar crystalline structures

Fourier transform infrared spectroscopy was applied to study the structural and thermal properties of bovine brain galactocerebroside (GalCer) containing amide linked non-hydroxylated or alpha-hydroxy fatty acids (NFA- and HFA-GalCer, respectively). Over the temperature range 0-90 degrees C, both GalCer displayed complex thermal transitions, characteristic of polymorphic phase behavior. Upon heating, aqueous dispersions of NFA- and HFA-GalCer exhibited high order-disorder transition temperatures near 80 and 72 degrees C, respectively. En route to the chain melting transition, the patterns of the amide I band of NFA-GalCer were indicative of two different lamellar crystalline phases, whereas those of HFA-GalCer were suggestive of lamellar gel and crystalline bilayers. Cooling from the liquid-crystalline phase resulted in the formation of another crystalline phase of NFA-GalCer and a gel phase of HFA-GalCer, with a phase transition near 62 and 66 degrees C, respectively. Prolonged incubation of GalCer bilayers at 38 degrees C revealed conversions among lamellar crystalline phases (NFA-GalCer) or between lamellar gel and crystalline bilayer structures (HFA-GalCer). Spectral changes indicated that the temperature and/or time induced formation of the lamellar crystalline structures of NFA- and HFA-GalCer was accompanied by partial dehydration and by rearrangements of the hydrogen bonding network and bilayer packing mode of GalCer.

The effect of polychlorinated biphenyls on the uptake of dopamine and other neurotransmitters into rat brain synaptic vesicles

Studies have shown that polychlorinated biphenyls may affect cognitive functions both in human and also in experimental animals. One of the neurochemical parameters that is changed after exposure to these compounds is a reduction in the dopamine level in the brain, although the mechanism behind this reduction is not known. We have therefore investigated whether this reduction could be caused by an effect on vesicular uptake. ortho-Chlorinated biphenyls are found to be competitive inhibitors of dopamine transport into synaptic vesicles from rat brain with K(i) concentrations as low as 4 microM. In contrast, several nonortho-chlorinated biphenyls did not inhibit vesicular uptake. The inhibition was specific for dopamine, in that the uptake of glutamate and GABA was inhibited at higher PCB concentrations under identical conditions. The vesicular Mg-ATPase proton pump was also inhibited at higher concentrations of PCBs than the dopamine transport. Uptake of methylamine gave no indication of any disruption of the vesicular proton gradient. The inhibition of dopamine vesicular uptake by PCBs was competitive. Several of the ortho-PCBs also inhibited the binding of tetrabenazine, which is known to bind to a site close to the dopamine binding site, at the vesicular transporter. The results show that inhibition of vesicular uptake may contribute to the decrease of dopamine reported in nervous tissue after exposure to PCBs under different conditions.

Thermotropic behavior of galactosylceramides with cis-monoenoic fatty acyl chains

To define the thermotropic behavior of galactosylceramides (GalCer) containing cis monounsaturated acyl chains, N-X:1Delta(X-9) cis galactosylsphingosines (GalSph) were synthesized (where X=24, 22, 20, or 18) and investigated by differential scanning calorimetry (DSC). After hydration of dried glycolipid, aqueous dispersions were prepared by repetitive heating and freeze-thaw cycles. The DSC data clearly showed that introducing a single cis double bond into the acyl chain of GalCer lowers the transition temperature of the main endothermic peak and affects the kinetics of formation of various metastable and stable gel phases. More importantly, the data emphasize the role that double bond location in concert with acyl chain length play in modulating the thermotropic behavior of GalCers. In contrast to the 18:1 GalCer and 20:1 GalCer endotherms which remain unchanged after identical repetitive heating scans and low temperature incubations, the thermotropic responses of 22:1 GalCer and 24:1 GalCer depended directly upon incubation time at lower temperatures following a heating scan. Only after extended incubation (4-5 days) did the endotherms revert to behavior observed during the initial heating scan that followed sample preparation by cyclic heating and freeze-thaw methods. The extended incubation times required for 22:1 GalCer and 24:1 GalCer to assume their more stable packing motifs appear to be consistent with nucleation events that promote transbilayer interdigitation. Yet, due to the slow kinetics of the process, the presence of cis monounsaturation in very long acyl chains that are common to GalCer may effectively inhibit transbilayer lipid interdigitation under physiological conditions.

Ionic fluxes through myelin membrane vesicles

Technical difficulties and a lack of reproducibility in procedures aimed at the production of myelin vesicles have delayed functional studies on membrane transport through myelin. Myelin vesicles could provide an excellent model for the study of the transport of ions and water, etc., across this type of membrane. They could also help improve our understanding of the molecular functions of the myelin sheath. In this investigation, a novel, nonaggressive method of producing central nervous system myelin vesicles is presented. Purified bovine myelin was incubated with iminodiacetic acid (an insoluble chelating agent that is easy to remove and does not interfere with further functional assays), and rendered insoluble on 1% crosslinked polystyrene beads (Chelex-100). Myelin vesicles obtained were impermeable to sugars (sucrose, glucose, and galactose), but showed a degree of permeability towards potassium salts as determined by light-scattering. Further experiments with fluorescent probes revealed an electrogenic K+ influx, as measured by oxonol V fluorescence quenching, and a significant H+ permeability measured using the pH-sensitive probe acridine orange. H+ permeability was not detected in control liposomes made from the same endogenous myelin lipids without protein. The results are discussed with reference to previous studies performed using purified myelin proteins in reconstituted systems. The relevance of these results with respect to ionic transport across myelin membrane is discussed.

Bilayer nanotubes and helical ribbons formed by hydrated galactosylceramides: acyl chain and headgroup effects

The molecular basis of bilayer tubule formation in hydrated galactosylceramide (GalCer) dispersions has been investigated by synthesizing different chain-pure GalCers and examining their aqueous mesomorphic phase structure by freeze fracture and negative-stain electron microscopy. Thermotropic characterization of the GalCer species by differential scanning calorimetry provided supplementary information that verified the phase state under which morphological observations were carried out. Under aqueous conditions and at room temperature, N-24:1 delta 15(cis) GalSph, the predominant monounsaturated, nonhydroxy acyl species of bovine brain GalCer (NFA-GalCer), formed cylindrical mesomorphic self-assemblies consisting almost exclusively of "nanotubes," i.e., lipid bilayer tubules of relatively uniform length and diameter (length, 250-400 nm; diameter, 25-30 nm). In contrast, N-24:0 GalSph, the major saturated, nonhydroxy acyl species of bovine brain GalCer, displayed no tendency to form these relatively small "nanotubes." Rather, N-24:0 GalSph formed larger, variable-length ribbon-like structures (length, 5,000-10,000 nm) that often appeared to undulate and, occasionally, appeared to be helically twisted. Interestingly, bovine brain GalCer, which contains high levels of the N-24:1 delta 15(cis) and N-24:0 species as well as 2-hydroxy acyl chains, formed multilamellar liposomes of variable size and showed little tendency to form cylindrical structures. This result suggested that changes to the polar interface/headgroup region imparted by the 2-hydroxy acyl species strongly influenced bilayer tubule and cylinder formation in GalCer. To define this influence more clearly, other sphingoid-based and glycerol-based lipids were investigated. Morphological characterization of N-24:1 delta 15(cis) sphingosylphosphorylcholine (24:1 SM) revealed no evidence of bilayer cylinder or tubule formation. Similar results were obtained with aqueous dispersions of 1-palmitoyl-2-nervonoyl phosphatidylcholine (16:0, 24:1 PC). Hence, the bulkier, more hydrated, zwitterionic phosphocholine headgroup inhibited the formation of bilayer nanotubes and cylinders under physiological saline conditions.

Macro-ripple phase formation in bilayers composed of galactosylceramide and phosphatidylcholine

As determined by freeze fracture electron microscopy, increasing levels of bovine brain galactosylceramide (GalCer) altered the surface structure of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers by inducing a striking "macro-ripple" phase in the larger, multilamellar lipid vesicles at GalCer mole fractions between 0.4 and 0.8. The term "macro-ripple" phase was used to distinguish it from the P beta' ripple phase observed in saturated, symmetric-chain length phosphatidylcholines. Whereas the P beta' ripple phase displays two types of corrugations, one with a wavelength of 12-15 nm and the other with a wavelength of 25-35 nm, the macro-ripple phase occurring in GalCer/POPC dispersions was of one type with a wavelength of 100-110 nm. Also, in contrast to the extended linear arrays of adjacent ripples observed in the P beta' ripple phase, the macro-ripple phase of GalCer/POPC dispersions was interrupted frequently by packing defects resulting from double dislocations and various disclinations and, thus, appeared to be continuously twisting and turning. Control experiments verified that the macro-ripple phase was not an artifact of incomplete lipid mixing or demixing during preparation. Three different methods of lipid mixing were compared: a spray method of rapid solvent evaporation, a sublimation method of solvent removal, and solvent removal using a rotary evaporation apparatus. Control experiments also revealed that the macro-ripple phase was observed regardless of whether lipid specimens were prepared by either ultra-rapid or manual plunge freezing methods as well as either in the presence or absence of the cryo-protectant glycerol. The macro-ripple phase was always observed in mixtures that were fully annealed by incubation above the main thermal transition of both POPC and bovine brain GalCer before rapid freezing. If the GalCer mixed with POPC contained only nonhydroxy acyl chains or only 2-hydroxy acyl chains, then the occurrence of macro-ripple phase decreased dramatically.