Nycodenz Gradient Research Articles

Presence of heterogeneous peroxisomal populations in the rat nervous tissue

Peroxisomes were purified from the nervous tissue of 14-day-old rats by means of a Nycodenz gradient. Peroxisomal enzymes exhibited different sedimentation patterns: dihydroxyacetone phosphate acyl-transferase equilibrates at 1.142 g/ml together with the first peak of catalase; palmitoyl-CoA oxidase and d-amino acid oxidase activities are mainly recovered at 1.154 g/ml; the second peak of catalase is found at 1.175 g/ml. Morphological and semi-quantitative analyses of immunogold-labelled peroxisomes reveal profound heterogeneity of the particles. Very small (=0.2 μm diameter), electron dense vesicles containing catalase or thiolase, but devoid of other tested enzymes, are preferentially found in the light region, together with larger (>0.2 <0.3 μm) and less electron dense palmitoyl-CoA oxidase-positive peroxisomes. At intermediate density (1.154 g/ml) peroxisomes of more uniform size (0.25–0.27 μm), containing palmitoyl-CoA oxidase or thiolase with or without catalase are preferentially found. This population extends toward the densest region of the gradient, where very large d-amino acid oxidase-containing peroxisomes are also found. In this region, smaller peroxisomes, often polymorphic, which are catalase- and thiolase-positive and d-amino acid oxidase/palmitoyl-CoA oxidase-negative, are also observed. The possibility that the heterogeneity of neural peroxisomes may reflect both cellular heterogeneity and ongoing peroxisomal biogenesis is discussed.

Periacinar stellate shaped cells in rat pancreas: identification, isolation, and culture

Background—The pathogenesis of pancreatic fibrosis is unknown. In the liver, stellate cells (vitamin A storing cells) play a significant role in the development of fibrosis.Aims—To determine whether cells resembling hepatic...

Wild-Type and Mutant Huntingtins Function in Vesicle Trafficking in the Secretory and Endocytic Pathways

Huntingtin is a cytoplasmic protein that is found in neurons and somatic cells. In patients with Huntington's disease (HD), the NH2-terminal region of huntingtin has an expanded polyglutamine tract. An abnormal protein interaction by mutant huntingtin has been proposed as a mechanism for HD pathogenesis. Huntingtin associates with vesicle membranes and interacts with proteins involved in vesicle trafficking. It is unclear where along vesicle transport pathways wild-type and mutant huntingtins are found and whether polyglutamine expansion affects this localization. To distinguish wild-type and mutant huntingtin, fibroblasts from normals and HD patients with two mutant alleles (homozygotes) were examined. Immunofluorescence confocal microscopy showed that mutant huntingtin localized with clathrin in membranes of thetransGolgi network and in clathrin-coated and noncoated endosomal vesicles in the cytoplasm and along plasma membranes. Separation of organelles in Nycodenz gradients showed that in normal and HD homozygote patient cells, huntingtin was present in membrane fractions enriched in clathrin. Similar results were obtained in fibroblasts from heterozyote juvenile HD patients who had a highly expanded polyglutamine tract in the HD allele. Western blot analysis of membrane fractions from rat brain showed that wild-type huntingtin was present in fractions that contained purified clathrin-coated membranes or a mixture of clathrin-coated and noncoated membranes. Electron microscopy of huntingtin immunoreactivity in rat brain revealed labeling along dendritic plasma membranes in association with clathrin-coated pits and clusters of noncoated endosomal vesicles 40–60 nm in diameter. These data suggest that wild-type and mutant huntingtin can influence vesicle transport in the secretory and endocytic pathways through associations with clathrin-coated vesicles.

Sorting of Lysosomal Membrane Glycoproteins lamp-1 and lamp-2 into Vesicles Distinct from Mannose 6-Phosphate Receptor/γ-Adaptin Vesicles at the trans-Golgi Network

Newly synthesized lysosomal membrane glycoproteins lamp-1 and lamp-2 are primarily sorted at the trans-Golgi network (TGN) by recognition of a tyrosine-based signal sequence in their cytoplasmic tails. It is presently unclear how this signal is recognized and what type of vesicle transports lamp-1 and lamp-2. Here, we describe a method to generate transport vesicles containing lamp proteins from the TGN in vitro. The method is based on incorporation of radioactive sialic acid in glycoproteins at the TGN by incubation of membranes with tritiated CMP-sialic acid. The generation of vesicles from labeled membranes required ATP and cytosol, and was temperature-dependent and brefeldin A-sensitive. Analysis on Nycodenz gradients revealed that lamp-vesicles were distinct from vesicles containing gamma-adaptin and mannose 6-phosphate receptor (MPR). Moreover, both these types of vesicles migrated differently than vesicles containing proteins destined for the plasma membrane. The conclusion that lamps and MPRs are sorted into different vesicles was further strengthened by the finding that whereas wortmannin both in vitro and in vivo inhibited the production of gamma-adaptin/MPR-containing vesicles, this drug had no effect on the generation of lamp-vesicles and on the sorting of lamps. The results indicate that membrane proteins containing tyrosine-based motifs for sorting at the TGN are segregated from clathrin-coated vesicles containing MPRs.

Intracellular Localization of Interleukin-6 in Eosinophils From Atopic Asthmatics and Effects of Interferon γ

AbstractEosinophils, prominent cells in asthmatic inflammation, have been shown to synthesize, store, and release an array of up to 18 cytokines and growth factors, including interleukin-6 (IL-6). In this report, we show that IL-6 immunofluorescence localizes to the matrix of the crystalloid granule in peripheral blood eosinophils from atopic asthmatics using confocal laser scanning microscopy (CLSM). Granule localization of IL-6 was confirmed using dot-blot analysis and enzyme-linked immunosorbent assay (ELISA) on subcellular fractions of highly purified eosinophils produced from density centrifugation across a 0%to 45%Nycodenz gradient. IL-6 was found to coelute with eosinophil crystalloid granule marker proteins, including eosinophil peroxidase (EPO), major basic protein (MBP), arylsulfatase B, and β-hexosaminidase. Immunoreactivity to IL-6 colocalized with granule-associated IL-2 and IL-5 in subfractionated eosinophils. We also made the novel and compelling observation that interferon γ (IFNγ), a Th1-type cytokine, stimulated an early elevation in eosinophil IL-6 immunoreactivity. A 2.5-fold enhancement of IL-6 immunoreactivity in eosinophil granules was observed within 10 minutes of IFNγ treatment (500 U/mL), as determined by subcellular fractionation and CLSM. These findings suggest that IFNγ has short-term effects on human eosinophil function and imply that a physiologic role exists for Th1-type cytokine modulation of Th2-type responses in these cells.

Intracellular Localization of Interleukin-6 in Eosinophils From Atopic Asthmatics and Effects of Interferon γ

Eosinophils, prominent cells in asthmatic inflammation, have been shown to synthesize, store, and release an array of up to 18 cytokines and growth factors, including interleukin-6 (IL-6). In this report, we show that IL-6 immunofluorescence localizes to the matrix of the crystalloid granule in peripheral blood eosinophils from atopic asthmatics using confocal laser scanning microscopy (CLSM). Granule localization of IL-6 was confirmed using dot-blot analysis and enzyme-linked immunosorbent assay (ELISA) on subcellular fractions of highly purified eosinophils produced from density centrifugation across a 0% to 45% Nycodenz gradient. IL-6 was found to coelute with eosinophil crystalloid granule marker proteins, including eosinophil peroxidase (EPO), major basic protein (MBP), arylsulfatase B, and β-hexosaminidase. Immunoreactivity to IL-6 colocalized with granule-associated IL-2 and IL-5 in subfractionated eosinophils. We also made the novel and compelling observation that interferon γ (IFNγ), a Th1-type cytokine, stimulated an early elevation in eosinophil IL-6 immunoreactivity. A 2.5-fold enhancement of IL-6 immunoreactivity in eosinophil granules was observed within 10 minutes of IFNγ treatment (500 U/mL), as determined by subcellular fractionation and CLSM. These findings suggest that IFNγ has short-term effects on human eosinophil function and imply that a physiologic role exists for Th1-type cytokine modulation of Th2-type responses in these cells.

Influence of acetylcholinesterase on embryonic spinal rat motoneurones growth in culture: a quantitative morphometric study.

Rat spinal motoneurones sampled at day embryonic 15 were purified using a Nycodenz gradient and cultured in defined medium, during 7 days, on glass coverslips coated with poly-L-lysine and laminine. Purified acetylcholinesterase (AChE), ecothiopate, BW 284C51 and fasciculin II, inhibitors of either the catalytic or peripheral site of AChE, were added to the defined medium. Morphological changes of spinal motoneurones were measured using a statistical quantitative morphometric method, allowing the determination of various parameters such as the number of neurites and bifurcations, the length of neurites, the surface and spreading index. Presence of AChE in the medium (4 units/mL) increases the surface and the total length of neurites and axons without any change in the spreading index. When spinal motoneurones were cultured on AChE coated substrate, neurones rapidly migrate and form clusters. Addition of ecothiopate (10(-6) M) in the medium, which selectively blocks the catalytic site of AChE, leads to a slight increase in the number of primary neurite and a decrease of the spreading index during the three first days in culture. BW 284C51 (10(-5) M) which blocks the catalytic site but also affect the peripheral one, significantly reduces the number of primary neurites and increases the number of bifurcations. Fasciculin II, a potent blocker (10(-9)M) of the AChE peripheral site induces a decrease of both primary neurites and bifurcations with a significant increase of the length and growth velocity of the axon, giving a drastic enhancement of the spreading index. These phenomena are discussed in terms of catalytic and non-catalytic function of AChE, including the involvement of the enzyme in adhesive processes, occurring during growth and differentiation of spinal motoneurones.

Detection of endothelin-1 and its receptors in rat liver endothelial cells by in situ reverse transcriptase-polymerase chain reaction

Background/Aims: Endothelin (ET)-1 is a potent vasoconstrictor, also in the liver; it increases intrahepatic resistance by targeting hepatic stellate cells acting via the ET-A receptor. The role of the expression and regulation of ET-1 and its receptors in endothelial liver cells, an important site of ET production in normal liver, is not yet well documented. In the present study, we have developed an in situ reverse transcriptase (RT)-polymerase chain reaction method to elucidate the presence of prepro-ET-1, ET-A and ET-B receptors in isolated rat liver endothelial cells. Methods: After in situ collagenase-pronase liver perfusion, endothelial cells were isolated using a Nycodenz gradient. RT-polymerase chain reaction and in situ RT-polymerase chain reaction were performed using specific primers for prepro-endothelin, ET-A and ET-B. Results: Trypan blue exclusion test showed a viability of more than 90% in the freshly isolated non-parenchymal cells. RT-polymerase chain reaction showed expression of prepro-endothelin, ET-A and ET-B in RNA isolated from the endothelial cells. After in situ RT-polymerase chain reaction, we detected cytoplasmatic staining representing the presence of mRNA in the endothelial cells, with all sets of primers. About 60–80% of cells were positive. Negative controls, in which the RT step was omitted, did not show any cytoplasmatic staining. Conclusions: Endothelin 1 and both ET-A and ET-B receptors are expressed in rat endothelial liver cells. The expression of ET and its receptors in liver endothelial cells could be important in the development of liver diseases and the regulation in microvascular exchange.

An endothelial storage granule for tissue-type plasminogen activator.

In previous studies we have shown that, after stimulation by a receptor ligand such as thrombin, tissue-type plasminogen activator (tPA) and von Willebrand factor (vWf) will be acutely released from human umbilical vein endothelial cells (HUVEC). However, the mechanisms involved in the secretion of these two proteins differ in some respects, suggesting that the two proteins may be stored in different secretory granules. By density gradient centrifugation of rat lung homogenates, a particle was identified that contained nearly all tPA activity and antigen. This particle had an average density of 1.11-1.12 g/ml, both in Nycodenz density gradients and in sucrose density gradients. A similar density distribution of tPA was found for a rat endothelial cell line and for HUVEC. After thrombin stimulation of HUVEC to induce tPA secretion, the amount of tPA present in high-density fractions decreased, concomitant with the release of tPA into the culture medium and a shift in the density distribution of P-selectin. vWf, known to be stored in Weibel-Palade bodies, showed an identical distribution to tPA in Nycodenz gradients. In contrast, the distribution in sucrose gradients of vWf from both rat and human lung was very different from that of tPA, suggesting that tPA and vWf were not present in the same particle. Using double-immunofluorescence staining of HUVEC, tPA- and vWf-containing particles showed a different distribution by confocal microscopy. The distribution of tPA also differed from the distribution of tissue factor pathway inhibitor, endothelin-1, and caveolin. By immunoelectronmicroscopy, immunoreactive tPA could be demonstrated in small vesicles morphologically different from the larger Weibel-Palade bodies. It is concluded that tPA in endothelial cells is stored in a not-previously-described, small and dense (d = 1.11-1.12 g/ml) vesicle, which is different from a Weibel-Palade body.

The hemopoietic growth factor, interleukin-3, promotes glucose transport by increasing the specific activity and maintaining the affinity for glucose of plasma membrane glucose transporters.

Most mammalian cells rely on an external supply of glucose for survival, proliferation, and function. Glucose enters cells through specific transporter molecules at the plasma membrane by a facilitative process that does not expend energy. Regulation of glucose transport into cells is thought to occur largely through transporter expression at the cell surface, but the extent to which the intrinsic properties of glucose transporters are regulated is at present controversial. Using a bone marrow-derived cell line that responds to the hemopoietic growth factor, interleukin-3 (IL-3), we investigated IL-3 regulation of glucose transport. IL-3 significantly increased 2-deoxyglucose (2-DOG) uptake within 1 h (26 +/- 8.0%, n = 11) with a maximum 73% increase after 6 h. Withdrawal of IL-3 resulted in decreased uptake within 1 h and this continued to decline to 43% of initial uptake by 16 h. To determine whether these changes in 2-DOG uptake were associated with corresponding changes in glucose transporter expression, subtype-specific antisera against Glut-1 and Glut-3 were used. Little change in membrane expression of these transporters was observed prior to 16 h. Fractionation of cell membranes on Nycodenz gradients showed that the majority of each transporter subtype was associated with the plasma membrane (63-93%) and that transporter distribution did not change markedly in response to addition or withdrawal of IL-3. These results demonstrate that IL-3 regulates glucose uptake by modulating the intrinsic transporting ability of glucose transporters. Decreased transporter affinity for 2-DOG and 3-O-methylglucose was observed following IL-3 withdrawal. Similar affinity changes were observed with 2-DOG following exposure of IL-3-stimulated cells to the protein kinase inhibitors, genistein and staurosporine. In contrast, the tyrosine phosphatase inhibitor, vanadate, acted like IL-3 to increase transporter affinity for glucose. Together these results demonstrate that IL-3 acts to maintain the intrinsic transport properties of glucose transporters without markedly affecting their expression or translocation.

Heterogeneity of pituitary folliculo-stellate cells: implications for interleukin-6 production and accessory function in vitro.

The population of folliculo-stellate (FS) cells of the rat anterior pituitary has been shown to be ultrastructurally and immunohistochemically heterogeneous. Based on the overlap of ultrastructural characteristics, the localization in the anterior pituitary and the co-expression within the same cel of the S-100 protein (a marker for FS cells) and MHC-class II determinants (an immune marker) we concluded that a partial overlap exists between the population of FS cells and the monocyte-derived dendritic cells (DC). In this report we describe that interleukin-6 (IL-6) immunoreactivity was found in situ in stellate cells of the rat, mouse and human anterior pituitary at a very low density (< 1% of the cells); the topography was reminiscent of the distribution of FS cells. In the present study we also analyse three different pituitary cell separation methods, in order to study the functional heterogeneity of the FS cells in vitro, and to verify whether functionally distinct subpopulations exist within the FS cell group. Production of bioactive IL-6 was measured in conditioned media of rat anterior pituitary cells separated by (i) bovine serum albumin (BSA) gradient sedimentation at 1 g, (ii) Nycodenz gradient and (iii) a magnetic cell separation (MACS) technique. Production of bioactive IL-6 by cell cultures of 1 to 4 days was correlated with the proportional number of S100 immunoreactive and S100 producing cells, but was not correlated with the proportional number of MHC-class II expressing (OX6-positive) dendritic cells (DC). The distribution pattern of OX6-positive DC was found to partly overlap with the distribution pattern of S100-positive cells in the BSA gradient. Co-sedimentation of S100-positive FS cells and MHC-class II-expressing DC was not restricted to the top fractions of the BSA gradient, but was also found in the low density Nycodenz fraction. MACS separation of the rat anterior pituitary cells resulted into a population enriched in OX6 and OX62 positive DC and a population devoid of such cells, while S100+ cells were equally divided into these two subpopulations. Although there was a significantly decreased production of IL-6 as compared to that of an original pituitary cell population, both MACS separated populations were equal in IL-6 production. The diminution in IL-6 production in both populations may be the result of an impediment of paracrine communication due to the MACS separation into these two populations. Our data also show that a subpopulation of FS cells was capable of stimulating T cell proliferation in vitro. Concomitantly with the distribution pattern of S100- and OX6-immunoreactive cells in the BSA and Nycodenz gradient fractions, we found a similar pattern of stimulation of T cell proliferation. Unlike the IL-6 production pattern, the T cell stimulating capacity was present in the MHC-class II-enriched cell population but absent in the MHC-class II-depleted cell population. These findings-together with earlier in situ histochemical data-suggest that there is an OX6+ S100- subpopulation of FS cells in the anterior pituitary that in itself is capable of stimulating T cell proliferation in vitro, and acts as lymphoid DC. There is also an S100+ OX6- population that is unable to stimulate T cell proliferation in vitro. Both populations are able to produce IL-6, but probably need stimuli from other subpopulations of pituitary cells (or exogeneous stimuli) to produce maximal amounts of IL-6.

Effect of gemfibrozil on centrifugal behavior of rat peroxisomes and activities of peroxisomal enzymes involved in lipid metabolism.

The effect of gemfibrozil, and analogue of clofibric acid, on the centrifugal behavior of peroxisomes and activity of peroxisomal enzymes involved in lipid metabolism was studied. Rats were fed chow containing 0.2% gemfibrozil for 2 weeks. Nycodenz gradient centrifugation of the light mitochondrial fraction revealed that peroxisomes of gemfibrozil-treated rats were concentrated in fractions of higher density compared with control rats. The activity of fatty acyl-CoA oxidase, crotonase, beta-hydroxybutyryl-CoA dehydrogenase, and thiolase (individual enzymes of the peroxisomal fatty acid beta-oxidation system) were enhanced 9.6, 2.3, 3.4 and 9.1 times respectively compared with controls, by treatment. The hydroxymethylglutaryl-CoA (HMG-CoA) reductase (rate-limiting enzyme of cholesterol synthesis) activity of peroxisomes and microsomes was greatly increased by in vivo treatment with gemfibrozil, but was decreased by addition of the agent to the assay mixture of the enzyme. Gemfibrozil directly inhibited the reductase activity and did so at a lower concentration than clofibric acid. Peroxisomal reductase was more resistant to damage by the agent than the microsomal enzyme. The HMG-CoA reductase activity of peroxisomes and microsomes of hyperlipidemic rats was also increased by in vivo treatment with gemfibrozil, whereas the serum cholesterol level was hardly changed. These results indicate that the effect of gemfibrozil differs from that of clofibric acid, the main difference being the effect on HMG-CoA reductase. Gemfibrozil increased reductase activity in vivo, unlike clofibric acid, but inhibited the enzyme in vitro to a greater extent than clofibric acid.

Biosynthesis and degradation of hyaluronan by nonparenchymal liver cells during liver regeneration

Hepatic stellate cells (HSC) and endothelial cells of the liver sinusoids synthesize and degrade hyaluronan, respectively. The roles of these cell types in the biosynthesis and degradation of hyaluronan were studied during regeneration following partial hepatectomy. Pure cultures of HSC and liver endothelial cells (LEC) were obtained from regenerating liver at different stages using a Nycodenz gradient followed by discontinuous Percoll gradient. The HSC that established 3 or 4 days after partial hepatectomy synthesized large amounts of hyaluronan when cultured in the presence of fetal calf serum (FCS) or platelet-derived growth factor B-chain homodimer (PDGF)-BB. These cells, as well as LEC, expressed active PDGF beta-receptors. Furthermore, the ability of LEC to degrade hyaluronan was decreased at early stages of liver regeneration. The increased synthesis of hyaluronan by HSC and the failure of LEC to catabolize the polysaccharide resulted in elevated hyaluronan concentrations in the blood. (Hepatology 1996 Jun;23(6):1650-5)

Biosynthesis and degradation of hyaluronan by nonparenchymal liver cells during liver regeneration.

Hepatic stellate cells (HSC) and endothelial cells of the liver sinusoids synthesize and degrade hyaluronan, respectively. The roles of these cell types in the biosynthesis and degradation of hyaluronan were studied during regeneration following partial hepatectomy. Pure cultures of HSC and liver endothelial cells (LEC) were obtained from regenerating liver at different stages using a Nycodenz gradient followed by discontinuous Percoll gradient. The HSC that established 3 or 4 days after partial hepatectomy synthesized large amounts of hyaluronan when cultured in the presence of fetal calf serum (FCS) or platelet-derived growth factor B-chain homodimer (PDGF)-BB. These cells, as well as LEC, expressed active PDGF beta-receptors. Furthermore, the ability of LEC to degrade hyaluronan was decreased at early stages of liver regeneration. The increased synthesis of hyaluronan by HSC and the failure of LEC to catabolize the polysaccharide resulted in elevated hyaluronan concentrations in the blood.

Labeling of peroxisomes with green fluorescent protein in living P. pastoris cells.

We exploited the light-activated fluorescent properties of the green fluorescent protein (GFP) of the jellyfish Aequorea victoria for studies on the peroxisomal sorting of polypeptides. GFP and GFP-SKL (containing a C-terminal, tripeptide peroxisomal targeting signal, SKL) were expressed from a methanol-inducible, alcohol oxidase (AOX1) promoter in the methylotrophic yeast Pichia pastoris. GFP was cytosolic, whereas the GFP-SKL fusion protein was targeted to peroxisomes, as demonstrated by biochemical fractionation of organelles on Nycodenz gradients. Neither GFP nor GFP-SKL affected the viability of yeast cells but both were fluorescent on excitation with 395-nm UV light. The subcellular locations of GFP and GFP-SKL in living yeast cells were monitored by fluorescence microscopy and their fluorescence was coupled to photo-oxidation of diaminobenzidine (DAB), resulting in the deposition of electron-dense oxidized DAB at intracellular locations of GFP derivatives. This photooxidation procedure permitted facile ultrastructural localization of GFP in cells by electron microscopy, and provided further evidence that GFP produced in P. pastoris is cytosolic, whereas GFP-SKL is peroxisomal. The GFP-SKL fusion protein is therefore a versatile reporter for the peroxisomal compartment, with many applications for studies involving peroxisomal import and biogenesis.

PHOSPHOLIPID TRANSFER ACTIVITIES ARE ENRICHED IN CYTOSOLIC FRACTIONS OF FRESHLY ISOLATED TYPE II CELLS (TII) COMPARED WITH WHOLE LUNG. † 2111

The phospholipid (PL) transfer proteins are cytosolic proteins that have been characterized by their ability to facilitate the transfer of PL's between membranes in vitro. In mammalian tissues including lung, 3 transfer proteins with different PL specificities have been identified: 1) phosphatidylcholine transfer protein (PC-TP) specific for PC; 2) phosphatidylinositol transfer protein (PI-TP), specific for PI and PC and to lesser extent PG, and 3) nonspecific lipid transfer protein (nsL-TP) which transfers all diacylPLs and cholesterol. In lung, a PL-TP specific for PG has been identified. Because recent evidence suggests that these proteins are involved in regulating phospholipid synthesis and membrane lipid composition, we determined by a vesicle-rat lung mitochondria transfer assay the transfer activities in cytosol (cyt) fractions from adult whole lung and TII from fetal, newborn, and adult rat lung freshly isolated by Nycodenz gradient centrifugation. Transfer activities for all PLs were enriched in adult TII cyt compared with whole lung (12.5-fold (PG), 9.2-fold (PI), 6.5-fold (PC) and 6.6-fold (PE)) (p newborn>adult TII. In cyts from cells from different stages, the transfer rate of PG>PI>PC>PE. The relative changes in activities of the PL-TPs during lung development may affect membrane lipid composition and hence, membrane functions. We are currently investigating the role of the PL-TPs in TII PL synthesis. Funded by NIH R29 HL44853.

ROLE OF PHOSPHATIDYLINOSITOL TRANSFER PROTEIN IN TYPE II PNEUMOCYTES: REGULATION OF CHOLINEPHOSPHATE CYTIDYLYLTRANSFERASE. † 2112

Phosphatidylinositol transfer protein (PI-TP) is one of a group of proteins that are cytosolic proteins characterized by their ability to facilitate the transfer of phospholipids between membranes in vitro. It has recently been demonstrated that yeast PI-TP (SEC14p) inhibited Golgi membrane cholinephosphate cytidylyltransferase (CYTase) in vitro andin vivo (Skinner HB, et al. Proc. Natl. Acad. Sci. 1995, 92:112-116). We hypothesized that PI-TP regulates CYTase in rat alveolar type II cells (TII). TII cells were freshly isolated from adult rat lung by Nycodenz gradient centrifugation and cytosol (cyt) protein obtained by digitonin release and microsome (mc) fractions obtained by differential centrifugation. Fractions (50-100μg) were incubated with 0.5μg rrPI-TP(gift G.M. Helmkamp, Jr., Kansas, KS) or buffer in total volume of 1 ml at 37°C for 5 to 30 min. Two forms of rrPI-TP were used - one bound to phosphatidylglycerol (PG) and one bound to phosphatidylcholine (PC). Cytosolic CYTase was stimulated 2.1- and 7.9-fold by PG-rrPI-TP and PC-rrPI-TP, respectively. There was 1.2-fold greater stimulation of mc CYTase by PC-rrPI-TP than PG-rrPI-TP. Co-incubation with PI/PC (90:10, mol:mol) vesicles or chlorpromazine, a PI-TP inhibitor, abolished the stimulatory effect. Immunoprecipitation of PI-TP in TII cyt with rabbit anti-PI-TP antibody decreased CYTase activity 47%. These results suggest that 1) PI-TP stimulates TII CYTase; 2) the interaction of PI-TP with CYTase is dependent, in part, on the bound phospholipid; 3) lipid vesicles may compete with mc membrane for PI-TP. PI-TP may be an important regulator of CYTase activation and/or membrane binding. Funded by NIH R29 HL44853.

The PAL1 gene product is a peroxisomal ATP-binding cassette transporter in the yeast Saccharomyces cerevisiae.

The PAL1 gene was isolated using PCR and degenerate oligonucleotide primers corresponding to highly conserved amino acid sequence motifs diagnostic of the ATP-binding cassette domain of the superfamily of membrane-bound transport proteins typified by mammalian multidrug resistance transporter 1 and Saccharomyces cerevisiae Ste6. The deduced PAL1 gene product is similar in length to, has the same predicted topology as, and shares the highest degree of amino acid sequence identity with two human proteins, adrenoleukodystrophy protein and peroxisomal membrane protein (70 kD), which are both presumptive ATP-binding cassette transporters thought to be constituents of the peroxisomal membrane. As judged by hybridization of a PAL1 probe to isolated RNA and by expression of a PAL1-lacZ fusion, a PAL1 transcript was only detectable when cells were grown on oleic acid, a carbon source which requires the biogenesis of functional peroxisomes for its metabolism. A pal1delta mutant grew normally on either glucose- or glycerol-containing media; however, unlike PAL1+ cells (or the pal1delta mutant carrying the PAL1 gene on a plasmid), pal1delta cells were unable to grow on either a solid medium or a liquid medium containing oleic acid as the sole carbon source. Antibodies raised against a chimeric protein in which the COOH-terminal domain of Pal1 was fused to glutathione S-transferase specifically recognized a protein in extracts from wild-type cells only when grown on oleic acid; this species represents the PAL1 gene product because it was missing in pal1delta cells and more abundant in pal1delta cells expressing PAL1 from a multicopy plasmid. The Pal1 polypeptide was highly enriched in the organellar pellet fraction prepared from wild-type cells by differential centrifugation and comigrated upon velocity sedimentation in a Nycodenz gradient with a known component of the peroxisomal matrix, e-oxoacyl-CoA thiolase. As judged by both subcellular fractionation and indirect immunofluorescence, localization of 3-oxoacyl-CoA thiolase to peroxisomes was unchanged whether Pal1 was present, absent, or overexpressed. These findings demonstrate that Pal1 is a peroxisome-specific protein, that it is required for peroxisome function, but that it is not necessary for the biogenesis of peroxisomes or for the import of 3-oxoacyl-CoA thiolase (and at least two other peroxisomal matrix proteins).

In vitro import of proteins into mitochondria of Trypanosoma brucei and Leishmania tarentolae

In eukaryotic evolution, the earliest branch of organisms to have mitochondria are the trypanosomatids. Their mitochondrial biogenesis not only includes import of most proteins, but also, unlike in other organisms, import of the whole set of tRNAs. In order to investigate these processes, we devised novel procedures for the isolation of mitochondria from two trypanosomatid species: Trypanosoma brucei and Leishmania tarentolae. Isotonic cell lysis followed by equilibrium density centrifugation in Nycodenz gradients yielded mitochondrial fractions exhibiting a membrane potential. Furthermore, we have used these fractions to reconstitute import of mitochondrial matrix proteins in vitro. Energy-dependent uptake of an artificial precursor protein, containing a trypanosomal presequence attached to mouse dihydrofolate reductase and of yeast mitochondrial alcohol dehydrogenase could be demonstrated. The presequences of both proteins were processed in T. brucei whereas only the trypanosomal one was cleaved in L. tarentolae. Trypsin pretreatment abolished the ability of the mitochondria to import proteins, indicating the involvement of proteinaceous components at the surface of mitochondria.

Structural aspects of autophagy.

As a first step towards isolation of autophagic sequestering membranes (phagophores), we have purified autophagosomes from rat hepatocytes. Lysosomes were selectively destroyed by osmotic rupture, achieved by incubation of hepatocyte homogenates with the cathepsin C substrate glycyl-phenylalanyl-naphthylamide (GPN). Mitochondria and peroxisomes were removed by Nycodenz gradient centrifugation, and cytosol, microsomes and other organelles by rate sedimentation through metrizamide cushions. The purified autophagosomes were bordered by dual or multiple concentric membranes, suggesting that autophagic sequestration might be performed either by single autophagic cisternae or by cisternal stacks. Okadaic acid, a protein phosphatase inhibitor, disrupted the hepatocytic cytokeratin network and inhibited autophagy completely in intact hepatocytes, perhaps suggesting that autophagy might be dependent on intact intermediate filaments. Vinblastine and cytochalasin D, which specifically disrupted microtubules and microfilaments, respectively, had relatively little (25-30%) inhibitory effect on autophagic sequestration. In a cryo-ultrastructural study, the various autophagic-lysosomal vacuoles were immunogold-labelled, using the cytosolic enzyme superoxide dismutase as an autophagic marker, Lgp120 as a lysosomal membrane marker, and bovine serum albumin as an endocytic marker. Vinblastine (50 microM) was found to inhibit both autophagic and endocytic flux into the lysosomes, with a consequent reduction in lysosomal size. Asparagine (20 mM) caused swelling of the lysosomes, probably as a result of the ammonia formation that could be observed at this high asparagine concentration. Autophagosomes and amphisomes (autophagic-endocytic, prelysosomal vacuoles) accumulated in asparagine-treated cells, reflecting an inhibition of autophagic flux that might be a consequence of lysosomal dysfunction.