Exchange of phosphatidylcholine between small unilamellar liposomes and human plasma high-density lipoprotein involves exclusively the phospholipid in the outer monolayer of the liposomal membrane

By making use of the capacity of phospholipase A₂ to degrade selectively the phospholipid in the outer half of the lipid bilayer of small unilamellar phospholipid/cholesterol vesicles without affecting the retention of a vesicle-encapsulated solute, we demonstrated that the exchange of phosphatidylcholine between such vesicles and human high density lipoprotein involves exclusively the phosphatidylcholine present in the outer monolayer of the vesicle membrane.     

Relationship between fish kairomone concentration in a lake and phototactic swimming by Daphnia

Light-induced swimming behaviour of Daphnia can lead to dielvertical migration. When this occurs, Daphnia may escape frompredation by juvenile 0+ fish. For this to happen, swimmingin response to the change in light intensity at dawn and duskmust be enhanced. This enhanced swimming reaction can be elicitedby fish-associated kairomone in the laboratory. We studied theeffect of these fish-associated kairomones in Lake Maarsseveen,The Netherlands. A bioassay was conducted in which light-inducedswimming reactions of Daphnia were used to quantify the kairomonesignal strength. In two successive years, 1998 and 1999, waterfrom Lake Maarsseveen was tested weekly in early summer. Sampleswere taken from the epilimnion at 3 m depth and from the hypolimnionat 15 m depth. Simultaneously, we caught 0+ perch (Perca fluviatilis)with bongo nets to determine its abundance. The length and weightof individuals caught in 1998 were determined to establish alength–weight relationship. Using this relationship andthe information on perch density from the trawls, the relativefish biomass was calculated for these two years. It was shownthat water from the epilimnion layer increasingly enhanced light-inducedswimming reactions until the second week of June, then the effectgradually disappeared. Water from the hypolimnion had no sucheffect. In 1998, these changes in signal strength correlatedwith the relative biomass of the 0+ perch, but in 1999, themaximum of the enhancement lagged 2 weeks behind the maximumof the biomass of the 0+ perch. This lag may be due to a differentdevelopment of the thermocline. We conclude that kairomone concentrationmay well correlate with 0+ perch biomass and thereby might informDaphnia not only about the presence, but also about the abundance,of juvenile perch.     

The dcr gene family of Desulfovibrio: Implications from the sequence of dcrH and phylogenetic comparison with other mcp genes

Desulfovibrio vulgaris Hildenborough contains a family of genes for methyl-accepting chemotaxis proteins (MCPs). Here we report the complete sequence of the gene for Desulfovibrio chemoreceptor H (dcrH). The deduced amino acid sequence of DcrH protein, which has an enlarged N-terminal, ligand binding domain, indicates a structure similar to that of other MCPs. Comparison of the sequences for DcrA, determined earlier, and DcrH indicated that similarity is essentially limited to the C-terminal excitation region. The dcr gene family differs, in this respect, from mcp gene families in other eubacteria (e.g. Escherichia coli and Bacillus subtilis), where MCPs share significant homology throughout their C-terminal signal transduction domains. This may point to an ancient evolutionary origin of the dcr gene family, which is widely distributed throughout the genus Desulfovibrio. The evolutionary origin of mcp genes was traced by comparing nucleotide sequences for the excitation region that is common to all MCPs. Phylogenetic analysis of sequences for thirty mcp genes from nine eubacterial and one archaebacterial species suggested that multiplication of mcp genes has occurred at least twice since the eubacteria diverged from the archaebacteria.Nucleotide accession number: The nucleotide sequence reported in this paper has been entered into GenBank under accession number U30319. Phone: 403-220-6388. Fax: 403-289-9311. Electronic mail address: voordouw@acs.ucalgary.ca.     

The involvement of parenchymal,Kupffer and endothelial liver cells in the hepatic uptake of intravenously injected liposomes effects of lanthanum and gadolinium salts

¹²⁵I-labeled albumin or poly(vinyl pyrrolidone) encapsulated in intermediate size multilamellar or unilamellar liposomes with 30–40% of cholesterol were injected intravenously into rats. In other experiments liposomes containing phosphatidyl[Me-¹⁴C]choline were injected. 1 h after injection parenchymal or non-parenchymal cells were isolated. Non-parenchymal cells were separated by elutriation centrifugation into a Kupffer cell fraction and an endothelial cell fraction. From the measurements of radioactivities in the various cell fractions it was concluded that the liposomes are almost exclusively taken up by the Kupffer cells. Endothelial cells did not contribute at all and hepatocytes only to a very low extent to total hepatic uptake of the ¹²⁵I-labels. Of the ¹⁴C-label, which orginates from the phosphatidylcholine moiety of the liposomes, much larger proportions were recovered in the hepatocytes. A time-dependence study suggested that besides the involvement of phosphatidylcholine exchange between liposomes and high density lipoprotein, a process of intercellular transfer of lipid label from Kupffer cells to the hepatocytes may be involved in this phenomenon. Lanthanum or gadolinium salts, which effectively block Kupffer cell activity, failed to accomplish an increase in the fraction of liposomal material recovered in the parenchymal cells. This is compatible with the notion that liposomes of the type used in these experiments have no, or at most very limited, access to the liver parenchyma following their intravenous administration to rats.     

Dietary practices,site-specific tumors,and age-at-death relationships

When rats are given freedom of dietary choice, the quantity and composition of the diet selected varies from animal to animal, but the risk to the individual of developing a variety of neoplasms is greatly increased over that of rats fed fixed diets. The increase in frequency of tumors cannot be explained on the basis of the amount or the composition of the diet selected. However, the application of “variable selection” statistical procedures shows that there are combinations of temporal-specific interacting dietary and dietary-dependent variables that are closely linked with cancer mortality. The multi-variable statistical model for animals dying with one type of tumor is not applicable to those dying with tumors of other types or sites. In every case, the only data required to characterize the dates of death of tumor-bearing individuals are those describing the dietary practices and growth responses prior to maturity. The model for animals with pancreatic tumors is used to illustrate the extent to which dietary practices of early life are linked with the animals’ survival.     

Lactosylceramide-induced stimulation of liposome uptake by Kupffer cells in vivo

Incorporation of 8 mol percent lactosylceramide into small unilamellar vesicles consisting of cholesterol and sphingomyelin in an equimolar ratio and containing [³H]inulin as a marker resulted in an increase in total liver uptake and a drastic change in intrahepatic distribution of the liposomes after intravenous injection into rats. The control vesicles without glycolipid accumulated predominantly in the hepatocytes, but incorporation of the glycolipid resulted in a larger stimulation of Kupffer-cell uptake (3.2-fold) than of hepatocyte uptake (1.2-fold). Liposome preparations both with and without lactosylceramide in which part of the sphingomyelin was replaced by phosphatidylserine, resulting in a net negative charge of the vesicles, were cleared much more rapidly from the blood and taken up by the liver to higher extents. The negative charge had, however, no influence on the intrahepatic distributions. The fast hepatic uptake of the negatively charged liposomes allowed competition experiments with substrates for the galactose receptors on liver cells. Inhibition of blood clearance and liver uptake of lactosylceramide-containing liposomes by $\text{N-}\text{acetyl-}\text{d}\text{-galactosamine}$ indicated the involvement of specific recognition sites for the liposomal galactose residues. This inhibitory effect of $\text{N-}\text{acetyl-}\text{d}\text{-galactosamine}$ was shown to be mainly the result of a decreased liposome uptake by the Kupffer cells, compatible with the reported presence of a galactose specific receptor on this cell type (Kolb-Bachofen et al. (1982) Cell 29, 859–866). The difference between the results on sphingomyelin-based liposomes as described in this paper and those on phosphatidylcholine-based liposomes as published previously (Spanjer and Scherphof (1983) Biochim. Biophys. Acta 734, 40–47) are discussed.