Interaction of mammalian and plant H+/sucrose transporters with 14-3-3 proteins.

The solute carrier 45 family (SLC45) was defined in the course of the Human Genome Project and consists of four members, A1-A4, which show only 20-30% identity of amino acid sequences among each other. All these members exhibit an identity of ∼20% to plant H+/sucrose cotransporters. Recently, we expressed members of the murine SLC45 family in yeast cells and demonstrated that they are, like their plant counterparts, H+/sucrose cotransporters. In contrast with the plant proteins, SLC45 transporters recognise also the monosaccharides glucose and fructose as physiological substrates and seem to be involved in alternative sugar supply as well as in osmoregulation of several mammalian tissues. In the present study, we provide novel insights into the regulation of SLC45 transporters. By screening for interaction partners, we found a 14-3-3 protein as a promising candidate for control of transport activity. Indeed, co-expression of the gamma isoform of murine 14-3-3 protein in yeast and Xenopus oocytes led to a significant decrease in transport rates of the murine SLC45 transporters as well as of the plant H+/sucrose transporter Sut1.

Putative role of an SLC45 H+/sugar cotransporter in mammalian spermatozoa.

In the present study, we describe the detection and analysis of a novel type of sugar transporter in mammalian spermatozoa. This transporter belongs to the SLC45 family for which two features are remarkable and distinguish it from other known families of sugar transporters. Firstly, SLC45 transporters recognise not only the monosaccharides glucose or fructose but also the disaccharide sucrose as a substrate. Secondly, the uptake of sugars is coupled to a proton gradient. Uptake experiments using radioactively labelled sucrose indicated a functional transporter of the SLC45 family in bull spermatozoa. Real-time PCR as well as Western blots demonstrated the occurrence of the SLC45 member A4 in mouse testis and sperms. Furthermore, immunocytochemical analysis of mouse tissues revealed that the signal of SLC45A4 was mainly located in the principle piece of spermatozoa. We postulate that the SLC45A4 transporter plays an important role in nutrition of spermatozoa during their maturation in the epididymis. Moreover, we suggest that knowledge about the presence of the SLC45A4 may be useful also for the methodical improvement of cryopreservation of mammalian spermatozoa.

Putative role of the H(+)/sucrose symporter SLC45A3 as an osmolyte transporter in the kidney.

The solute carrier family 45 a3 member (SLC45A3), known also as prostein, has been implicated with prostate cancer and the regulation of lipid metabolism in oligodendrocytes. Recently, we expressed SLC45A3 in yeast cells and characterised it as a proton-coupled sucrose symporter. However, the physiological functions of SLC45A3 were still unknown. Here, we report that SLC45A3 occurs in the kidney and is highly expressed in the medullary collecting duct (IMCD), a part of the kidney responsible for final urine concentration and faced to hyperosmotic environment. Moreover, messenger RNA (mRNA) expression of endogenous SLC45A3 in rat IMCD cells as well as in NRK52E cells increased up to four-fold under hyperosmotic conditions at 600 mOsmol/kg. Using NRK52E cells as an experimental model, we investigated the proton-coupled sugar transport and found that the uptake of sucrose or glucose was enhanced by hyperosmolarity. Down-regulation of expression by small interfering RNA (siRNA) decreased the osmotically inducible part of sucrose uptake and confirmed the involvement of SLC45A3 in this process. Furthermore, we observed an up to four-fold elevation of sucrose uptake triggered by hyperosmolarity across the apical membrane of NRK52E cells, while uptake across the basolateral membrane was not affected. Due to this finding, we conclude that SLC45A3 may occur at the luminal side of kidney epithelial cells and thus may take up solutes from the tubular fluid. Altogether, we show that SLC45A3 is a novel sugar transporter in kidney and hypothesise that the disaccharide sucrose, and probably the monosaccharides glucose and fructose, may serve as compatible osmolytes in urine.

The biological basis for poly-L-lactic acid-induced augmentation.

BACKGROUND: Granulomatous reactions to poly-L-lactic acid (PLLA)-based filler have been described previously. Neither the biological background of these partly late-onset reactions or the desired augmenting effect of PLLA has been studied to date. Histological studies have revealed foreign body reactions and foreign body giant cell formation. OBJECTIVE: The aim of this study was to increase our knowledge about the biological mechanisms behind the augmenting effect of PLLA-based filler. METHODS: We characterised the cell infiltrate and collagen type of PLLA-treated tissue by immunofluorescence staining. The expression of genes related to collagen metabolism was determined. RESULTS: CD68(+) macrophages were found next to PLLA. CD90(+) fibroblasts were found alongside. αSMA-positive structures indicated myofibroblasts and neovascularisation. Substantial collagen type III deposition was detected next to PLLA particles and collagen type I was found at the periphery of PLLA encapsulations. mRNA expression for collagen type I and III transcripts, as well as for TGFß1 and TIMP1, was upregulated significantly. CONCLUSION: PLLA-induced augmentation is most likely based on capsule formation orchestrating macrophages, (myo-)fibroblasts, and collagen type I and III fibres. We observed considerably slower degradation of PLLA particles than described previously. Thus PLLA particles were still retrievable 28 months after subcutaneous application.

Proton-associated sucrose transport of mammalian solute carrier family 45: an analysis in Saccharomyces cerevisiae.

The members of the solute carrier 45 (SLC45) family have been implicated in the regulation of glucose homoeostasis in the brain (SLC45A1), with skin and hair pigmentation (SLC45A2), and with prostate cancer and myelination (SLC45A3). However, apart from SLC45A1, a proton-associated glucose transporter, the function of these proteins is still largely unknown, although sequence similarities to plant sucrose transporters mark them as a putative sucrose transporter family. Heterologous expression of the three members SLC45A2, SLC45A3 and SLC45A4 in Saccharomyces cerevisiae confirmed that they are indeed sucrose transporters. [(14)C]Sucrose-uptake measurements revealed intermediate transport affinities with Km values of approximately 5 mM. Transport activities were best under slightly acidic conditions and were inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone, demonstrating an H(+)-coupled transport mechanism. Na(+), on the other hand, had no effect on sucrose transport. Competitive inhibition assays indicated a possible transport also of glucose and fructose. Real-time PCR of mouse tissues confirmed mRNA expression of SLC45A2 in eyes and skin and of SLC45A3 primarily in the prostate, but also in other tissues, whereas SLC45A4 showed a predominantly ubiquitous expression. Altogether the results provide new insights into the physiological significance of SLC45 family members and challenge existing concepts of mammalian sugar transport, as they (i) transport a disaccharide, and (ii) perform secondary active transport in a proton-dependent manner.

Reversible disassembly of the yeast V-ATPase revisited under in vivo conditions.

Primary active proton transport by eukaryotic V-ATPases (vacuolar ATPases) is regulated via the reversible disassembly of the V1Vo holoenzyme into its peripheral catalytic V1 complex and its membrane-bound proton-translocating Vo complex. This nutrient-dependent phenomenon had been first detected in the midgut epithelium of non-feeding moulting tobacco hornworms (Manduca sexta) and in glucose-deprived yeast cells (Saccharomyces cerevisiae). Since reversible disassembly to date had been investigated mostly in vitro, we wanted to test this phenomenon under in vivo conditions. We used living yeast cells with V-ATPase subunits fused to green, yellow or cyan fluorescent protein and found that only the V1 subunit C (Vma5) was released into the cytosol after substitution of extracellular glucose with galactose, whereas the other V1 subunits remained at or near the membrane. FRET analysis demonstrated close proximity between V1 and Vo even under glucose-starvation conditions. Disassembly, but not reassembly, depended on functional microtubules. Results from overlay blots, pull-down assays and bimolecular fluorescence complementation support the assumption that subunit C interacts directly with microtubules without involvement of linker proteins.

Subunit positioning and stator filament stiffness in regulation and power transmission in the V1 motor of the Manduca sexta V-ATPase.

The vacuolar H(+)-ATPase (V-ATPase) is an ATP-driven proton pump essential to the function of eukaryotic cells. Its cytoplasmic V1 domain is an ATPase, normally coupled to membrane-bound proton pump Vo via a rotary mechanism. How these asymmetric motors are coupled remains poorly understood. Low energy status can trigger release of V1 from the membrane and curtail ATP hydrolysis. To investigate the molecular basis for these processes, we have carried out cryo-electron microscopy three-dimensional reconstruction of deactivated V1 from Manduca sexta. In the resulting model, three peripheral stalks that are parts of the mechanical stator of the V-ATPase are clearly resolved as unsupported filaments in the same conformations as in the holoenzyme. They are likely therefore to have inherent stiffness consistent with a role as flexible rods in buffering elastic power transmission between the domains of the V-ATPase. Inactivated V1 adopted a homogeneous resting state with one open active site adjacent to the stator filament normally linked to the H subunit. Although present at 1:1 stoichiometry with V1, both recombinant subunit C reconstituted with V1 and its endogenous subunit H were poorly resolved in three-dimensional reconstructions, suggesting structural heterogeneity in the region at the base of V1 that could indicate positional variability. If the position of H can vary, existing mechanistic models of deactivation in which it binds to and locks the axle of the V-ATPase rotary motor would need to be re-evaluated.

The SLC45 gene family of putative sugar transporters.

According to the classic point of view, transport of sugars across animal plasma membranes is performed by two families of transporters. Secondary active transport occurs via Na(+) symporters of the SLC5 gene family, while passive transport occurs via facilitative transporters of the SLC2 family. In recent years a new family appeared in the scenery which was called the SLC45 gene family of putative sugar transporters, mainly because of obvious similarities to plant sucrose transporters. The SLC45 family consists of only four members that have been denominated A1-A4. These members apparently have counterparts in all vertebrates. Moreover, their amino acid sequences reveal close homologies also to respective invertebrate proteins such as a recently detected sucrose transporter in Drosophila, and suggest a phylogenetic relationship also to corresponding proteins from plants, fungi and bacteria. This minireview describes the molecular features of its members with a focus on their possible role as sugar transporters.

Vacuolar H(+)-ATPases: intra- and intermolecular interactions.

V-ATPases in eukaryotes are heteromultimeric, H(+)-transporting proteins. They are localized in a multitude of different membranes and energize many different transport processes. Unique features of V-ATPases are, on the one hand, their ability to regulate enzymatic and ion transporting activity by the reversible dissociation of the catalytic V(1) complex from the membrane bound proton translocating V(0) complex and, on the other hand, their high sensitivity to specific macrolides such as bafilomycin and concanamycin from streptomycetes or archazolid and apicularen from myxomycetes. Both features require distinct intramolecular as well as intermolecular interactions. Here we will summarize our own results together with newer developments in both of these research areas.

Identification of an animal sucrose transporter.

According to a classic tenet, sugar transport across animal membranes is restricted to monosaccharides. Here, we present the first report of an animal sucrose transporter, SCRT, which we detected in Drosophila melanogaster at each developmental stage. We localized the protein in apical membranes of the late embryonic hindgut as well as in vesicular membranes of ovarian follicle cells. The fact that knockdown of SCRT expression results in significantly increased lethality demonstrates an essential function for the protein. Experiments with Saccharomyces cerevisiae as a heterologous expression system revealed that sucrose is a transported substrate. Because the knockout of SLC45A2, a highly similar protein belonging to the mammalian solute carrier family 45 (SLC45) causes oculocutaneous albinism and because the vesicular structures in which SCRT is located appear to contain melanin, we propose that these organelles are melanosome-like structures and that the transporter is necessary for balancing the osmotic equilibrium during the polymerization process of melanin by the import of a compatible osmolyte. In the hindgut epithelial cells, sucrose might also serve as a compatible osmolyte, but we cannot exclude the possibility that transport of this disaccharide also serves nutritional adequacy.

Vacuolar-type proton pumps in insect epithelia.

Active transepithelial cation transport in insects was initially discovered in Malpighian tubules, and was subsequently also found in other epithelia such as salivary glands, labial glands, midgut and sensory sensilla. Today it appears to be established that the cation pump is a two-component system of a H(+)-transporting V-ATPase and a cation/nH(+) antiporter. After tracing the discovery of the V-ATPase as the energizer of K(+)/nH(+) antiport in the larval midgut of the tobacco hornworm Manduca sexta we show that research on the tobacco hornworm V-ATPase delivered important findings that emerged to be of general significance for our knowledge of V-ATPases, which are ubiquitous and highly conserved proton pumps. We then discuss the V-ATPase in Malpighian tubules of the fruitfly Drosophila melanogaster where the potential of post-genomic biology has been impressively illustrated. Finally we review an integrated physiological approach in Malpighian tubules of the yellow fever mosquito Aedes aegypti which shows that the V-ATPase delivers the energy for both transcellular and paracellular ion transport.

Stimulus-induced phosphorylation of vacuolar H(+)-ATPase by protein kinase A.

Eukaryotic vacuolar-type H(+)-ATPases (V-ATPases) are regulated by the reversible disassembly of the active V(1)V(0) holoenzyme into a cytosolic V(1) complex and a membrane-bound V(0) complex. The signaling cascades that trigger these events in response to changing cellular conditions are largely unknown. We report that the V(1) subunit C of the tobacco hornworm Manduca sexta interacts with protein kinase A and is the only V-ATPase subunit that is phosphorylated by protein kinase A. Subunit C can be phosphorylated as single polypeptide as well as a part of the V(1) complex but not as a part of the V(1)V(0) holoenzyme. Both the phosphorylated and the unphosphorylated form of subunit C are able to reassociate with the V(1) complex from which subunit C had been removed before. Using salivary glands of the blowfly Calliphora vicina in which V-ATPase reassembly and activity is regulated by the neurohormone serotonin via protein kinase A, we show that the membrane-permeable cAMP analog 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (8-CPT-cAMP) causes phosphorylation of subunit C in a tissue homogenate and that phosphorylation is reduced by incubation with antibodies against subunit C. Similarly, incubation of intact salivary glands with 8-CPT-cAMP or serotonin leads to the phosphorylation of subunit C, but this is abolished by H-89, an inhibitor of protein kinase A. These data suggest that subunit C binds to and serves as a substrate for protein kinase A and that this phosphorylation may be a regulatory switch for the formation of the active V(1)V(0) holoenzyme.

The V-ATPase subunit C binds to polymeric F-actin as well as to monomeric G-actin and induces cross-linking of actin filaments.

Previously, we have shown that the V-ATPase holoenzyme as well as the V1 complex isolated from the midgut of the tobacco hornworm (Manduca sexta) exhibits the ability of binding to actin filaments via the V1 subunits B and C (Vitavska, O., Wieczorek, H., and Merzendorfer,H. (2003) J. Biol. Chem. 278, 18499-18505). Since the recombinant subunit C not only enhances actin binding of the V1 complex but also can bind separately to F-actin, we analyzed the interaction of recombinant subunit C with actin. We demonstrate that it binds not only to F-actin but also to monomeric G-actin. With dissociation constants of approximately 50 nm, the interaction exhibits a high affinity, and no difference could be observed between binding to ATP-G-actin or ADP-G-actin, respectively. Unlike other proteins such as members of the ADF/cofilin family, which also bind to G- as well as to F-actin, subunit C does not destabilize actin filaments. On the contrary, under conditions where the disassembly of F-actin into G-actin usually occurred, subunit C stabilized F-actin. In addition, it increased the initial rate of actin polymerization in a concentration-dependent manner and was shown to cross-link actin filaments to bundles of varying thickness. Apparently bundling is enabled by the existence of at least two actin-binding sites present in the N- and in the C-terminal halves of subunits C, respectively. Since subunit C has the possibility to dimerize or even to oligomerize, spacing between actin filaments could be variable in size.

The insect plasma membrane H+ V-ATPase: intra-, inter-, and supramolecular aspects.

The plasma membrane H+ V-ATPase from the midgut of larval Manduca sexta, commonly called the tobacco hornworm, is the sole energizer of epithelial ion transport in this tissue, being responsible for the alkalinization of the gut lumen up to a pH of more than 11 and for any active ion movement across the epithelium. This minireview deals with those topics of our recent research on this enzyme that may contribute novel aspects to the biochemistry and physiology of V-ATPases. Our research approaches include intramolecular aspects such as subunit topology and the inhibition by macrolide antibiotics, intermolecular aspects such as the hormonal regulation of V-ATPase biosynthesis and the interaction of the V-ATPase with the actin cytoskeleton, and supramolecular aspects such as the interactions of V-ATPase, K+/H+ antiporter, and ion channels, which all function as an ensemble in the transepithelial movement of potassium ions.

A novel role for subunit C in mediating binding of the H+-V-ATPase to the actin cytoskeleton.

Primary proton transport by V-ATPases is regulated via the reversible dissociation of the V(1)V(0) holoenzyme into its V(1) and V(0) subcomplexes. Laser scanning microscopy of different tissues from the tobacco hornworm revealed co-localization of the holoenzyme and F-actin close to the apical membranes of the epithelial cells. In midgut goblet cells, no co-localization was observed under conditions where the V(1) complex detaches from the apical membrane. Binding studies, however, demonstrated that both the V(1) complex and the holoenzyme interact with F-actin, the latter with an apparently higher affinity. To identify F-actin binding subunits, we performed overlay blots that revealed two V(1) subunits as binding partners, namely subunit B, resembling the situation in the osteoclast V-ATPase (Holliday, L. S., Lu, M., Lee, B. S., Nelson, R. D., Solivan, S., Zhang, L., and Gluck, S. L. (2000) J. Biol. Chem. 275, 32331-32337), but, in addition, subunit C, which gets released during reversible dissociation of the holoenzyme. Overlay blots and co-pelleting assays showed that the recombinant subunit C also binds to F-actin. When the V(1) complex was reconstituted with recombinant subunit C, enhanced binding to F-actin was observed. Thus, subunit C may function as an anchor protein regulating the linkage between V-ATPase and the actin-based cytoskeleton.