The diversity of lipocalin receptors.

Lipocalins are important carriers of preferentially hydrophobic molecules, but they can also bind other ligands, like highly polar siderophores or intact proteins. Consequently, they are involved in a variety of physiological processes in many species. Since lipocalins are mainly extracellular proteins, they have to interact with cell receptors to exert their biological effects. In contrast to the large number of lipocalins identified in the last years, the number of receptors known is still limited. Nevertheless, some novel findings concerning the molecules involved in cellular uptake or signaling effects of lipocalins have been made recently. This review presents a detailed overview of the receptors identified so far. The methods used for isolation or identification are described and structural as well as functional information on these proteins is presented essentially in chronological order of their initial discovery.

Phage-display reveals interaction of lipocalin allergen Can f 1 with a peptide resembling the antigen binding region of a human γδT-cell receptor.

Although some progress has been achieved in understanding certain aspects of the allergenic mechanism of animal lipocalins, they still remain largely enigmatic. One possibility to unravel this property is to investigate their interaction with components of the immune system. Since these components are highly complex we intended to use a high-throughput technology for this purpose. Therefore, we used phage-display of a random peptide library for panning against the dog allergen Can f 1. By this method we identified a Can f 1 binding peptide corresponding to the antigen-binding site of a putative γδT-cell receptor. Additional biochemical investigations confirmed this interaction.

A role of heparan sulphate proteoglycan in the cellular uptake of lipocalins ß-lactoglobulin and allergen Fel d 4.

Lipocalins, small extracellular hydrophobic molecule carriers, can be internalized by a variety of different cells. However, to date receptors have only been identified for human lipocalins. Here, we specifically investigated uptake mechanisms for lipocalins ß-lactoglobulin and Fel d 4 in HeLa and Chinese hamster ovary (CHO) cells. We provide evidence that cell surface heparan sulphate proteoglycan is essential for internalization of these lipocalins. In HeLa cells, lipocalin uptake was inhibited by competition with soluble heparin, enzymatic digestion of cellular heparan sulphate by heparinase and inhibition of its biosynthesis by sodium chlorate. Biochemical studies by heparin affinity chromatography and colocalization studies further supported a role of heparan sulphate proteoglycan in lipocalin uptake. Finally, lipocalin uptake was blocked in CHO mutant cells defective in glycosaminoglycan biosynthesis whereas in wild-type cells it was clearly detectable. Thus, cell surface heparan sulphate proteoglycan represents a novel component absolutely participating in the cellular uptake of some lipocalins.

Apolipoprotein D Concentration in Human Plasma during Aging and in Parkinson's Disease: A Cross-Sectional Study.

Apolipoprotein D (ApoD), a lipocalin transporter of small hydrophobic molecules, plays an important role in several neurodegenerative diseases. ApoD is expressed in and secreted from a variety of peripheral and brain tissues. Increments of ApoD have been reported in relation with oxidative stress conditions, aging, and degeneration in the nervous system. Preliminary findings support the role of ApoD in neuroprotection. However, its role in PD remains unclear. To date, no studies have been performed on the relationship between ApoD in the blood and PD, as neurodegenerative pathology related to oxidative damage. We investigated the concentration of ApoD in the blood of healthy control subjects and PD patients with mild-to-moderate neurological impairment. ApoD plasma levels were measured using sandwich enzyme-linked immunosorbent assays (ELISA) in 90 healthy subjects (aging-analysis cohort) and in 66 PD patients at different stages compared with 19 age-matched healthy subjects. Significant age-related increase of ApoD was detected in subjects older than 65 years of age (p < 0.002). In PD patients, a significant increase in ApoD plasma concentration was found compared with healthy subjects of the same age (p < 0.05). ApoD and PD stage are significantly correlated (p < 0.05). ApoD might be a valid marker for the progression of PD.

A novel silver-containing absorbent wound dressing based on spacer fabric.

An ideal absorbent dressing requires high absorbency, moisture retention and antimicrobial properties. In this study, the application of silver-containing spacer fabric as an antimicrobial wound dressing was investigated. The silver distribution on the spacer fabric was different from that of layer-by-layered fabric. The middle layer of the spacer fabric contained much higher amounts of silver, while the layer-by-layered fabric had lower silver contents in its middle layers than its surface layers. The silver-containing spacer fabric could keep a better moist environment for the wound than commercial foam dressing. For the silver-containing spacer fabric, 100% reductions in viability were observed for both the Gram-positive Staphylococcus aureus and the Gram-negative Klebsiella pneumoniae after only 1 h. The spacer fabric could keep most of the silver in its middle layer and kill bacteria in the middle layer rather than at the wound contact surface. This way to absorb wound exudates and kill bacteria within the dressing reduces the silver concentration on the wound bed, and therefore this could be an efficient way to lower the potential of silver entering the human body, and prevent silver toxicity and accelerate wound healing.

Allergenic Can f 1 and its human homologue Lcn-1 direct dendritic cells to induce divergent immune responses.

Why and when the immune system skews to Th2 mediated allergic immune responses is still poorly characterized. With two homologous lipocalins, the major respiratory dog allergen Can f 1 and the human endogenous, non-allergenic Lipocalin-1, we investigated their impact on human monocyte-derived dendritic cells (DC). The two lipocalins had differential effects on DC according to their allergenic potential. Compared to Lipocalin-1, Can f 1 persistently induced lower levels of the Th1 skewing maturation marker expression, tryptophan breakdown and interleukin (IL)-12 production in DC. As a consequence, T cells stimulated by DC treated with Can f 1 produced more of the Th2 signature cytokine IL-13 and lower levels of the Th1 signature cytokine interferon-γ than T cells stimulated by Lipocalin-1 treated DC. These data were partially verified by a second pair of homologous lipocalins, the cat allergen Fel d 4 and its putative human homologue major urinary protein. Our data indicate that the crosstalk of DC with lipocalins alone has the potential to direct the type of immune response to these particular antigens. A global gene expression analysis further supported these results and indicated significant differences in intracellular trafficking, sorting and antigen presentation pathways when comparing Can f 1 and Lipocalin-1 stimulated DC. With this study we contribute to a better understanding of the induction phase of a Th2 immune response.

Antibacterial activity of rifamycins for M. smegmatis with comparison of oxidation and binding to tear lipocalin.

A mutant of Mycobacterium smegmatis is a potential class I model substitute for Mycobacterium tuberculosis. Because not all of the rifamycins have been tested in this organism, we determined bactericidal profiles for the 6 major rifamycin derivatives. The profiles closely mirrored those established for M. tuberculosis. Rifalazil was confirmed to be the most potent rifamycin. Because the tuberculous granuloma presents a harshly oxidizing environment we explored the effects of oxidation on rifamycins. Mass spectrometry confirmed that three of the six major rifamycins showed autoxidation in the presence of trace metals. Oxidation could be monitored by distinctive changes including isosbestic points in the ultraviolet-visible spectrum. Oxidation of rifamycins abrogated anti-mycobacterial activity in M. smegmatis. Protection from autoxidation was conferred by binding susceptible rifamycins to tear lipocalin, a promiscuous lipophilic protein. Rifalazil was not susceptible to autoxidation but was insoluble in aqueous solution. Solubility was enhanced when complexed to tear lipocalin and was accompanied by a spectral red shift. The positive solvatochromism was consistent with robust molecular interaction and binding. Other rifamycins also formed a complex with lipocalin, albeit to a lesser extent. Protection from oxidation and enhancement of solubility with protein binding may have implications for delivery of select rifamycin derivatives.

Targeting iron acquisition blocks infection with the fungal pathogens Aspergillus fumigatus and Fusarium oxysporum.

Filamentous fungi are an important cause of pulmonary and systemic morbidity and mortality, and also cause corneal blindness and visual impairment worldwide. Utilizing in vitro neutrophil killing assays and a model of fungal infection of the cornea, we demonstrated that Dectin-1 dependent IL-6 production regulates expression of iron chelators, heme and siderophore binding proteins and hepcidin in infected mice. In addition, we show that human neutrophils synthesize lipocalin-1, which sequesters fungal siderophores, and that topical lipocalin-1 or lactoferrin restricts fungal growth in vivo. Conversely, we show that exogenous iron or the xenosiderophore deferroxamine enhances fungal growth in infected mice. By examining mutant Aspergillus and Fusarium strains, we found that fungal transcriptional responses to low iron levels and extracellular siderophores are essential for fungal growth during infection. Further, we showed that targeting fungal iron acquisition or siderophore biosynthesis by topical application of iron chelators or statins reduces fungal growth in the cornea by 60% and that dual therapy with the iron chelator deferiprone and statins further restricts fungal growth by 75%. Together, these studies identify specific host iron-chelating and fungal iron-acquisition mediators that regulate fungal growth, and demonstrate that therapeutic inhibition of fungal iron acquisition can be utilized to treat topical fungal infections.

Gel electrophoretic analysis of corneal collagen after photodynamic cross-linking treatment.

PURPOSE: Photodynamic collagen cross-linking by using ultraviolet A (UVA) irradiation and the photosensitizer riboflavin has been recently introduced as a new possible treatment of progressive keratoconus. This is the first study, to our knowledge, investigating biochemical aspects of the new procedure. Its aim was to analyze the possible changes in the electrophoretic pattern of corneal collagen type I after collagen cross-linking treatment. METHODS: Twenty fresh postmortem porcine corneas were cross-linked; another 20 porcine corneas treated with physiologic saline were used as controls. After removal of the central 10 mm of the epithelium, the corneas were treated with the photosensitizer riboflavin and UVA irradiation for 30 minutes by using a double UVA diode (370 nm, 3 mW/cm). For biochemical analysis, the central 10-mm corneal buttons were trephined, tissue was homogenized, and collagen type I was extracted. Subsequently, the collagen extracts were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. RESULTS: In the controls, the typical collagen pattern of normal cornea was found with 1 gamma trimer band, 2 beta dimer bands, and 2 alpha monomer bands. In the cross-linked samples, there was an additional intense polymer band in the stacking gel that was resistant to mercaptoethanol, heat, and pepsin treatment. Its molecular size was estimated to be at least 1000 kDa. CONCLUSIONS: In the cross-linked corneas, a strong band of high-molecular-weight collagen polymers was shown as the biochemical correlate of the cross-linking effect, showing the efficiency of the new cross-linking procedure. This polymer band complies well with the morphologic correlate of an increased fiber diameter after cross-linking treatment. Its chemical stability supports hopes of a long-term effect of the new treatment.

Lipocalin-interacting-membrane-receptor (LIMR) mediates cellular internalization of beta-lactoglobulin.

beta-Lactoglobulin (BLG) is a member of the lipocalin protein family and a major food-borne allergen in humans. Numerous in vitro studies have suggested a role for BLG in molecular transport processes; however, its physiological role remains enigmatic. A cellular receptor for BLG has been proposed, but has not yet been identified. Here we show that human LIMR, known to act as an endocytic receptor for lipocalin-1, also binds bovine BLG and mediates its cellular uptake. The specificity of this interaction is corroborated by a complete block of cellular uptake of BLG in the presence of LIMR antibodies or LIMR downregulation by antisense RNA. Furthermore, heterologous expression of human LIMR in insect cells mediates cellular internalization of FITC-BLG. Since LIMR is highly expressed in the human intestine, it might also function in the uptake of food-borne BLG.

Development of a fast and reliable method for the assessment of microbial colonization and growth on textiles by DNA quantification.

There is a lack of relevant methods to assess the colonization of textiles by skin bacteria because present methods are mainly culture-based procedures. Therefore, the goal of this study was to develop a fast and sensitive culture-independent procedure for the quantification of microbial colonization and growth on textiles. We have established a suitable protocol to use DNA quantification as a reliable method for in vitroand in vivoinvestigations of textiles. For DNA extraction, a two-step procedure comprising treatment of the textile with a solution containing Triton X-100 and lysozyme for 1 h and a successive treatment by SDS and proteinase K for 2 h turned out to be most efficient. DNA extracted from textiles and fabrics was than quantified with the highly sensitive PicoGreen fluorescent dye. In vitrochallenge tests demonstrated a strong correlation between numbers of bacteria on textiles and amount of DNA extracted from textiles. Therefore, this method was used to compare different materials after in vivotrials for assessment of their susceptibility for microbial colonization and growth.

Oligomeric state of lipocalin-1 (LCN1) by multiangle laser light scattering and fluorescence anisotropy decay.

Multiangle laser light scattering and fluorescence anisotropy decay measurements clarified the oligomeric states of native and recombinant tear lipocalin (lipocalin-1, TL). Native TL is monomeric. Recombinant TL (5-68 microM) with or without the histidine tag shows less than 7% dimer formation that is not in equilibrium with the monomeric form. Fluorescence anisotropy decay showed a correlation time of 9-10 ns for TL (10 microM-1 mM). Hydrodynamic calculations based on the crystallographic structure of a monomeric TL mutant closely concur with the observed correlation time. The solution properties calculated with HYDROPRO and SOLPRO programs from the available crystallographic structure of a monomeric TL mutant concur closely with the observed fluorescence anisotropy decay. The resulting model shows that protein topology is the major determinant of rotational correlation time and accounts for deviation from the Stokes-Einstein relation. The data challenge previous gel filtration studies to show that native TL exists predominantly as a monomer in solution rather than as a dimer. Delipidation of TL results in a formation of a complex oligomeric state (up to 25%). These findings are important as the dynamic processes in the tear film are limited by diffusional, translational as well as rotational, properties of the protein.

Interaction of purified tear lipocalin with lipid membranes.

PURPOSE: The principal lipid-interacting protein in human tears has been reported to be tear lipocalin (Tlc). Tlc has been suggested to scavenge harmful lipophilic substances from the corneal epithelium and to maintain the integrity of the anterior tear film lipid layer by binding and releasing lipid(s) that are accommodated within the protein. Although lipids can be extracted from Tlc, it is still unclear whether Tlc can actually bind to lipid membranes and accept membrane lipids and whether it possesses lipid transfer activity. The purpose of this study was to explore the interaction of Tlc with neutral, anionic, and cationic lipid membranes and to assess the potential of Tlc to facilitate the transfer of either polar or neutral lipids in a lipid transfer assay. METHODS: The binding of Tlc to lipid membranes was assessed by a monolayer technique and fluorescence spectroscopy. The polar lipid transfer activity of Tlc was assessed with a radiometric assay based on the transfer of (14)C-phosphatidylcholine (PC) from PC-liposomes to HDL(3). The neutral lipid transfer activity of Tlc was assayed by measuring the transfer of radioactive cholesteryl ester from LDL to HDL(3). RESULTS: Purified Tlc showed significant surface activity as evidenced by an increase in surface pressure at the air-buffer interface. Likewise, it interacted actively with neutral, anionic, and cationic lipid monolayers, as evidenced by an equal increase in surface pressure despite the surface charge. Enhanced quenching of the single tryptophan residue of Tlc by pyrene and I(-) anion suggested that different protein domains are involved in the interaction of Tlc with oppositely charged lipid membranes. Finally, radiometric assays revealed that Tlc does not possess any neutral or polar lipid transfer activity between lipid vesicles or/and lipoproteins. CONCLUSIONS: Tlc interacted with lipid membranes composed of neutral, cationic, or anionic membranes, which supports a role for Tlc in the maintenance of the tear film interfaces. Tlc did not show any neutral or polar lipid transfer activity whatsoever. The findings suggest that the notion of the role of Tlc as the major lipid-transferring protein in human tears should be revised.

Low-temperature biodegradation of high amounts of phenol by Rhodococcus spp. and basidiomycetous yeasts.

Four cold-adapted microbial strains able to degrade high amounts of phenol were isolated from hydrocarbon-contaminated alpine soils. Two of the strains were bacteria identified as Rhodococcus spp., and two strains were basidiomycetous yeasts. One of the yeasts was identified as Trichosporon dulcitum, while the second yeast strain belonged to the Urediniomycetes and probably represents a novel species. This strain was not able to grow at temperatures above 20 degrees C, while the other three strains were cold-tolerant and could grow at temperatures ranging from 1-25 degrees C (T. dulcitum) or 1-30 degrees C (rhodococci). The yeast strains were characterized by a substantially lower optimum temperature for growth and biodegradation compared to the bacteria. The urediniomycete strain degraded 5 mM phenol at 1 degrees C faster than the two bacteria at 10 degrees C. The optimum temperature for phenol degradation was 10 degrees C (novel yeast species), 20 degrees C (T. dulcitum), or 30 degrees C (rhodococci). Using fed-batch cultivation in mineral medium with phenol as the sole carbon source, high amounts of phenol were degraded at 10 degrees C. Both rhodococci degraded up to 12.5 mM phenol, while the two yeast strains even utilized as much as 15 mM phenol.

The 1.8-A crystal structure of human tear lipocalin reveals an extended branched cavity with capacity for multiple ligands.

In contrast with earlier assumptions, which classified human tear lipocalin (Tlc) as an outlier member of the lipocalin protein family, the 1.8-A resolution crystal structure of the recombinant apoprotein confirms the typical eight-stranded antiparallel beta-barrel architecture with an alpha-helix attached to it. The fold of Tlc most closely resembles the bovine dander allergen Bos d 2, a well characterized prototypic lipocalin, but also reveals similarity with beta-lactoglobulin. However, compared with other lipocalin structures Tlc exhibits an extremely wide ligand pocket, whose entrance is formed by four partially disordered loops. The cavity deeply extends into the beta-barrel structure, where it ends in two distinct lobes. This unusual structural feature explains the known promiscuity of Tlc for various ligands, with chemical structures ranging from lipids and retinoids to the macrocyclic antibiotic rifampin and even to microbial siderophores. Notably, earlier findings of biological activity as a thiol protease inhibitor have no correspondence in the three-dimensional structure of Tlc, rather it appears that its proteolytic fragments could be responsible for this phenomenon. Hence, the present structural analysis sheds new light on the ligand binding activity of this functionally obscure but abundant human lipocalin.

Human tear lipocalin exhibits antimicrobial activity by scavenging microbial siderophores.

Human tear lipocalin (TL; also known as Lcn1) is a secretory protein present in large amounts in fluids that cover epithelial surfaces such as tears and respiratory secretions. It is supposed to act as a physiological scavenger of hydrophobic, potentially harmful molecules, but there is evidence that it also inhibits bacterial growth. In the present study, we reconsidered the possibility that TL might interfere with microbial growth by scavenging of siderophores, as described for human neutrophil gelatinase-associated lipocalin (NGAL). Indeed, our experiments revealed that TL binds to microbial siderophores with high affinities. In contrast to NGAL, which was shown to have some specificity for bacterial catecholate-type siderophores, TL binds to a broad array of siderophores, including bacterial catecholate-type enterobactin and hydroxamate-type desferrioxamine B, and all major classes of fungal siderophores. By adding exogenous TL, bacterial and fungal growth could be inhibited under iron-limiting conditions. Thus, TL might be a novel member of the innate immune system especially involved in mucosal defense against fungal infections.

Antisense down-regulation of lipocalin-interacting membrane receptor expression inhibits cellular internalization of lipocalin-1 in human NT2 cells.

There is increasing experimental evidence demonstrating that many lipocalins bind to specific cell surface receptors. However, whereas the binding of lipocalins to their lipophilic ligands has now been characterized in much detail, there is a lack of knowledge about the nature of lipocalin receptors, the physiological role of receptor binding, and the molecular mechanism of ligand delivery. We previously identified a novel human membrane protein (lipocalin-1-interacting membrane receptor (LIMR)), which interacts with lipocalin-1 (Wojnar, P., Lechner, M., Merschak, P., and Redl, B. (2001) J. Biol. Chem. 276, 20206-20212). In the present study, we investigated the physiological role of LIMR and found this protein to be essential for mediating internalization of lipocalin-1 (Lcn-1) in NT2 cells, leading to its degradation. Whereas control NT2 cells rapidly internalized (125)I-Lcn-1 or fluorescein isothiocyanate-labeled Lcn-1, NT2 cells that were made LIMR deficient by cDNA antisense expression greatly accumulated Lcn-1 in the culture medium but did not internalize it. Because sequence and structure analysis indicated that proteins similar to LIMR are present in several organisms and at least two closely related orthologues are found in human and mouse, we suggest LIMR to be the prototype of a new family of endocytic receptors, which are topographically characterized by nine putative transmembrane domains and a characteristic large central cytoplasmic loop.

Human lipocalin-1, a physiological scavenger of lipophilic compounds, is produced by corticotrophs of the pituitary gland.

Lipocalin-1 (Lcn-1), a member of the lipocalin superfamily that binds a broad array of different chemical classes of lipophilic ligands, is believed to act as a physiological scavenger of potentially harmful lipophilic molecules. Thus far, it was thought to be produced exclusively by a number of exocrine glands and tissues, including lachrymal and lingual glands, prostate, secretory glands of the tracheobronchial tract, and sweat glands. Using Northern blotting analysis, we were able to demonstrate Lcn-1 expression by the human pituitary gland. Moreover, double immunolabeling with antibodies against Lcn-1 and pituitary gland hormones and detection with fluorophore-conjugated secondary antibodies revealed that Lcn-1 is specifically produced by corticotrophs, clearly indicating that its distribution is not restricted to exocrine tissues.