Subcellular compartmentalization of saccharide moieties in cultured normal and malignant cells

Combined Lectin- and Immuno-histochemistry (CLIH) for Fluorescence Microscopy

The function of glycoproteins depends both on their polypeptide chain and sugar residues. For detection and localization of glycoproteins in tissue sections, methods of immunohistochemistry (IHC) and lectin histochemistry (LHC) are usually used separately. For a better understanding of the expression and distribution of variants of glycoproteins, tissue sections can be analyzed by combined lectin- and immuno-histochemistry (CLIH). CLIH exploits the advantages of both IHC and LHC and can therefore contribute to research in glycobiology and other fields of cell biology. Since cancer transformation is accompanied by alterations in the glycosylation of some glycoproteins, CLIH could also be exploited for improved classification of cancers. The chapter considers how CLIH could be employed on paraffin sections and semithin cryosections for fluorescence microscopy. Five different protocols of CLIH are described in detail as well as appropriate negative controls.

Stimulated emission depletion microscopy with a single depletion laser using five fluorochromes and fluorescence lifetime phasor separation

Stimulated emission depletion (STED) microscopy achieves super-resolution by exciting a diffraction-limited volume and then suppressing fluorescence in its outer parts by depletion. Multiple depletion lasers may introduce misalignment and bleaching. Hence, a single depletion wavelength is preferable for multi-color analyses. However, this limits the number of usable spectral channels. Using cultured cells, common staining protocols, and commercially available fluorochromes and microscopes we exploit that the number of fluorochromes in STED or confocal microscopy can be increased by phasor based fluorescence lifetime separation of two dyes with similar emission spectra but different fluorescent lifetimes. In our multi-color FLIM-STED approach two fluorochromes in the near red (exc. 594 nm, em. 600–630) and two in the far red channel (633/641–680), supplemented by a single further redshifted fluorochrome (670/701–750) were all depleted with a single laser at 775 nm thus avoiding potential alignment issues. Generally, this approach doubles the number of fully distinguishable colors in laser scanning microscopy. We provide evidence that eight color FLIM-STED with a single depletion laser would be possible if suitable fluorochromes were identified and we confirm that a fluorochrome may have different lifetimes depending on the molecules to which it is coupled.

Probes for Fluorescent Visualization of Specific Cellular Organelles

The defining characteristic of eukaryotic cells is the segregation of critical cellular functions within various membrane bound cellular organelles, including the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria. Cell biologists therefore have extensively utilized organelle specific counterstains to help identify the localization of specific proteins or other targets of interest in order to garner an understanding of either their potential functions or their effects on the cell. There currently is a wide array of fluorescent dyes and reagents that can be utilized in live and fixed cells to identify organelles, thereby creating challenges in both choosing between the plethora of options and optimizing their use. Here we present a discussion of commonly utilized commercially available organelle dyes and summarize the factors that influence selection of the various dyes for: a given organelle; live versus fixed cellular conditions; adaptation to a specific protocol; spectral multiplexing; or matching excitation/emission spectra to available imaging equipment. Also presented are recommended protocols for a typical example reagent that can be reliably utilized to visualize its target cellular organelle.

Selective fragmentation of the trans-Golgi apparatus by Rickettsia rickettsii

Fragmentation of the Golgi apparatus is observed during a number of physiological processes including mitosis and apoptosis, but also occurs in pathological states such as neurodegenerative diseases and some infectious diseases. Here we show that highly virulent strains of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, induce selective fragmentation of the trans-Golgi network (TGN) soon after infection of host cells by secretion of the effector protein Rickettsial Ankyrin Repeat Protein 2 (RARP2). Remarkably, this fragmentation is pronounced for the trans-Golgi network but the cis-Golgi remains largely intact and appropriately localized. Thus R. rickettsii targets specifically the TGN and not the entire Golgi apparatus. Dispersal of the TGN is mediated by the secreted effector protein RARP2, a recently identified type IV secreted effector that is a member of the clan CD cysteine proteases. Site-directed mutagenesis of a predicted cysteine protease active site in RARP2 prevents TGN disruption. General protein transport to the cell surface is severely impacted in cells infected with virulent strains of R. rickettsii. These findings suggest a novel manipulation of cellular organization by an obligate intracellular bacterium to determine interactions with the host cell.

Chiral nanoprobes for targeting and long-term imaging of the Golgi apparatus

The targeting and long-term imaging of the Golgi apparatus have been realized vial-cysteine functionalized nanoprobes.

The Golgi apparatus is an essential subcellular organelle. Targeting and monitoring the Golgi change at the single-cell level over a long time scale are critical but are challenges that have not yet been tackled. Inspired by the precise Golgi positioning ability of galactosyltransferase and protein kinase D, due to their cysteine residues, we developed a method for long-term Golgi imaging. Fluorescent molecules, carbon quantum dots (CQDs) and silica nanoparticles could target the Golgi when they are modified with l-cysteine. l-Cysteine-rich chiral carbon quantum dots (LC-CQDs), which have the benefits of a high Golgi specificity from l-cysteine and excellent photostability and biocompatibility from the CQDs, are proven to be highly suitable for long-term in situ imaging of the Golgi. Investigation of the mechanism showed that free thiol groups and the l-type stereo configuration of LC-CQDs are essential for specific targeting of the Golgi. With the aid of the as-prepared LC-CQDs, the dynamic changes of the Golgi in the early stage of viral infection were visualized. The Golgi targeting and imaging strategy used in this work is beneficial for Golgi-targeted drug delivery and early diagnosis and therapy of Golgi diseases.

A review of reagents for fluorescence microscopy of cellular compartments and structures, Part II: reagents for non-vesicular organelles

A wide range of fluorescent dyes and reagents exist for labeling organelles in live and fixed cells. Choosing between them can sometimes be confusing, and optimization for many of them can be challenging. Presented here is a discussion on the commercially-available reagents that have shown the most promise for each organelle of interest, including endoplasmic reticulum/nuclear membrane, Golgi apparatus, mitochondria, nucleoli, and nuclei, with an emphasis on localization of these structures for microscopy. Included is a featured reagent for each structure with a recommended protocol, troubleshooting guide, and example image.

Uptake and intracellular fate of fluorescent-magnetic glyco-nanoparticles

Iron oxide based nanoparticles are finding their way as leading actors in nanotechnology applications to medicine. Magnetite nanoparticles are currently being used in clinics for the detection of hepatic tumors based on their unspecific accumulation in liver. More and more works are being published on potential applications of magnetic nanoparticles in diagnostics and therapy. But the interaction between magnetic nanoparticles and human cells at the subcellular level is only now beginning to be studied and more basic research is needed in this field. This work studies the interaction between carbohydrate functionalized gold-coated magnetite nanoparticles and C33 tumoural human cells as the first step towards the in vivo application of these nanoparticles. The uptake of this magnetic material follows a similar trend to that described for other nanoparticles. The intracellular fate of these nanoparticles once internalized has been unveiled, and an automatic methodology for the analysis of co-localization data has also been tested and compared to a more classic approach. These results can help in the optimization of the design of magnetic nanoparticles depending on their final application.

A glutathione peroxidase 4 (GPx4) homologue from southern bluefin tuna is a secreted protein: first report of a secreted GPx4 isoform in vertebrates

The antioxidant enzyme glutathione peroxidase 4 (GPx4) is capable of reducing complex lipid hydroperoxides in addition to hydrogen peroxide and organic hydroperoxides. Mammals express three GPx4 isoforms that are targeted to nucleoli, mitochondria or cytosol via variable amino termini. To better understand the role of this important antioxidant enzyme in marine finfish, we determined the subcellular localisation of a GPx4 homologue from southern bluefin tuna (Thunnus maccoyii; SBT). We created constructs for the expression of the selenocysteine-to-cysteine mutant of SBT GPx4 (GPx4C) tagged with enhanced green fluorescent protein (EGFP), including or lacking a putative amino-terminal signal peptide, and expressed the fusion proteins in a fish cell line. Fluorescence microscopy revealed that the full-length GPx4C-EGFP fusion protein localised to the trans-Golgi, suggesting that tuna GPx4 may be directed to the secretory pathway. Anti-GFP immunoblotting of cell lysates and proteins from culture media showed that the secretion of SBT GPx4 into the culture medium required an amino-terminal signal peptide. According to available sequence data, the SBT GPx4 isoform studied here is representative of other piscine GPx4 isoforms, suggesting that the secretion of at least one GPx4 isoform may be common amongst teleost fish.

Calreticulin and focal-contact-dependent adhesion

Cell adhesion is regulated by a variety of Ca2+-regulated pathways that depend on Ca2+-binding proteins. One such protein is calreticulin, an ER-resident protein. Calreticulin signalling from within the ER can affect processes outside the ER, such as expression of several adhesion-related genes, most notably vinculin and fibronectin. In addition, changes in the expression level of calreticulin strongly affect tyrosine phosphorylation of cellular proteins, which is known to affect many adhesion-related functions. While calreticulin has been localized to cellular compartments other than the ER, it appears that only the ER-resident calreticulin affects focal-contact-dependent adhesion. In contrast, calreticulin residing outside the ER may be involved in contact disassembly and other adhesion phenomena. Here, we review the role of calreticulin in focal contact initiation, stabilization, and turnover. We propose that calreticulin may regulate cell-substratum adhesion by participating in an "ER-to-nucleus" signalling and in parallel "ER-to-cell surface" signalling based on posttranslational events.

Mouse RIC-3, an endoplasmic reticulum chaperone, promotes assembly of the alpha7 acetylcholine receptor through a cytoplasmic coiled-coil domain

RIC-3 (resistant to inhibitor of cholinesterase) is a transmembrane protein, found in invertebrates and vertebrates, that modulates the surface expression of a variety of nicotinic acetylcholine receptors (nAChRs) in neurons and other cells. To understand its mechanism of action, we investigated the cellular location, transmembrane topology and cellular mechanism by which RIC-3 facilitates alpha7 assembly and surface expression in cultured mammalian cells. We show that the mouse protein is targeted to the ER by the first 31 aa which act as a cleavable signal sequence. The mature protein is a single-pass type I transmembrane protein whose N terminus resides in the lumen of the ER with the coiled-coil domain in the cytoplasm. RIC-3, which binds both unfolded and folded alpha7 subunits, facilitates the surface expression of receptor principally by promoting the folding and assembly of the alpha7 subunits in the ER into fully polymerized receptor. Functional analysis shows that facilitation of surface expression of alpha7 in mammalian cells is reduced in RIC-3 mutants lacking the signal peptide, the lumenal segment or the coiled-coil domain, but not in mutants lacking the long C-terminal region downstream of the coiled-coil domain. We show that the coiled-coil domain of mRIC-3 is not required for the interaction of mRIC-3 with alpha7, but does mediate a homotypic interaction between molecules of mRIC-3. We suggest that efficient assembly of the homomeric alpha7 nAChR may thus require mRIC-3 self-association through the cytoplasmic coiled-coil domain and suggest a model by which this may occur.

Opposing effects of Ndel1 and alpha1 or alpha2 on cytoplasmic dynein through competitive binding to Lis1

Lis1 is an essential protein whose insufficiency causes aberrant neuronal positioning during neocortical development. It is believed to regulate both cytoplasmic dynein, a microtubule minus-end-directed motor, through direct interaction, and platelet-activating factor acetylhydrolase (PAF-AH) Ib by complexing with the catalytic subunits alpha1 and alpha2. Although alpha1 and alpha2 are highly expressed in brain, their deficiencies fail to cause brain abnormality. Here, we show that overexpression of alpha2 or alpha1 results in inactivation of dynein characterized by Golgi and endosome dispersion and mitotic delay. Further overexpression of Lis1 or Ndel1, a Lis1- and dynein-binding protein that is also crucial for dynein function, restored Golgi and endosome distribution. Biochemical assays showed that alpha1 and especially alpha2, were able to compete against Ndel1 and dynein for Lis1 binding in a dose-dependent manner. Overexpression of alpha2 in developing rat brain repressed the radial migration of neurons and mitotic progression of neuroprogenitors. By contrast, a Lis1-binding-defective point mutant, alpha2(E39D), was ineffective in the above assays. These results indicate an antagonistic effect of alpha1, alpha2 and Ndel1 for Lis1 binding, probably to modulate dynein functions in vivo. They also help to explain why brain development is particularly sensitive to a decrease in Lis1 levels.

Mucosal adjuvanticity of a Shigella invasin complex with dna-based vaccines

Protection against many infectious diseases may require the induction of cell-mediated and mucosal immunity. Immunization with plasmid DNA-based vaccines has successfully induced cell-mediated immune responses in small animals but is less potent in humans. Therefore, several methods are under investigation to augment DNA vaccine immunogenicity. In the current study, a mucosal adjuvant consisting of an invasin protein-lipopolysaccharide complex (Invaplex) isolated from Shigella spp. was evaluated as an adjuvant for DNA-based vaccines. Coadministration of plasmid DNA encoding the Orientia tsutsugamushi r56Karp protein with Invaplex resulted in enhanced cellular and humoral responses in intranasally immunized mice compared to immunization with DNA without adjuvant. Mucosal immunoglobulin A, directed to plasmid-encoded antigen, was detected in lung and intestinal compartments after Invaplex-DNA immunization followed by a protein booster. Moreover, immunization with Invaplex elicited Shigella-specific immune responses, highlighting its potential use in a combination vaccine strategy. The capacity of Invaplex to enhance the immunogenicity of plasmid-encoded genes suggested that Invaplex promoted the uptake and expression of the delivered genes. To better understand the native biological activities of Invaplex related to its adjuvanticity, interactions between Invaplex and mammalian cells were characterized. Invaplex rapidly bound to and was internalized by nonphagocytic, eukaryotic cells in an endocytic process dependent on actin polymerization and independent of microtubule formation. Invaplex also mediated transfection with several plasmid DNA constructs, which could be inhibited with monoclonal antibodies specific for IpaB and IpaC or Invaplex-specific polyclonal sera. The cellular binding and transport capabilities of Invaplex likely contribute to the adjuvanticity and immunogenicity of Invaplex.

Distinct transport and intracellular activities of two GlcAT-P isoforms

A neural glycotope, human natural killer-1 carbohydrate, is involved in synaptic plasticity. The key biosynthetic enzyme is a glucuronyltransferase, GlcAT-P, a type II membrane protein comprising an N-terminal cytoplasmic tail, transmembrane domain, stem region, and C-terminal catalytic domain. Previously, we reported that GlcAT-P has two isoforms differing in only the presence or absence of the N-terminal 13 amino acids (P-N13) in the cytoplasmic tail, but the functional distinction of these two isoforms has not been reported. Herein, we show that when expressed in Neuro2A cells, short form GlcAT-P (sGlcAT-P) exhibited significantly higher glucuronylation activity than the longer form (lGlcAT-P), despite their comparable specific activities in vitro. In addition, sGlcAT-P was strictly localized in Golgi apparatus, whereas lGlcAT-P was mainly localized in Golgi but partly in the endoplasmic reticulum. We demonstrated that the small GTPase, Sar1, recognized a dibasic motif in the cytoplasmic tail near P-N13 that was important for exiting the endoplasmic reticulum, and Sar1 interacted with sGlcAT-P more strongly than lGlcAT-P. Finally, the attachment of P-N13 to another glycosyltransferase, polysialyltransferase-I (ST8Sia-IV), had similar effects, such as reduced activity and entrapment within endoplasmic reticulum. These results suggest that P-N13 can control glycosyltransferase transport through Sar1 binding interference.

ApRab3, a biosynthetic Rab protein, accumulates on the maturing phagosomes and symbiosomes in the tropical sea anemone, Aiptasia pulchella

Symbiosome biogenesis and function are central to the endosymbiotic interaction between symbiotic dinoflagellates and their host cnidarians. To understand these important organelles, we have been conducting studies to identify and characterize symbiosome-associated proteins of the Rab family, key regulatory components of vesicular trafficking and membrane fusion in eukaryotic cells. Our prior studies have implicated three endocytic Rab proteins in the regulation of symbiosome biogenesis. Here, we show that ApRab3 is a new member of the Rab3 subfamily, associating with symbiosomes and accumulating on the maturing phagosomes in the A. pulchella digestive cells. ApRab3 is 78% identical to human Rab3C, and contains all Rab 3-specific signature motifs. EGFP-ApRab3-labeled vesicular structures tended to either align along the cell peripheral, or aggregate at one side of the nucleus. ApRab3 specifically co-distributed with the TGN marker, WGA, but not other organelle-specific markers tested. Immunofluorescence staining with a specific peptide antibody showed similar results. Significantly, an expression of a constitutively active mutant caused the enlargement and random dispersion of EGFP-ApRab3-decorated compartments in PC12 cells. Together, these data suggest that ApRab3 is a new member of the Rab3 subfamily, participating in the biosynthetic trafficking pathway, and symbiosome biogenesis involves an interaction with ApRab3-positive vesicles.

Analysis of the suitability of calreticulin inducible HEK cells for adhesion studies: microscopical and biochemical comparisons

Calreticulin is a Ca(2+)-buffering ER chaperone that also modulates cell adhesiveness. In order to study the effect of calreticulin on the expression of adhesion-related genes, we created a calreticulin inducible Human Embryonic Kidney (HEK) 293 cell line. We found that fibronectin mRNA and both intra- and extra-cellular fibronectin protein levels increased following calreticulin induction. However, despite this increase in fibronectin, HEK293 cells did not assemble an extracellular fibrillar fibronectin matrix regardless of the level of calreticulin expression. Furthermore, HEK293 cells exhibited a poorly organized actin cytoskeleton, did not have clustered fibronectin receptors at the cell surface, and did not form focal contacts. This likely accounts for the lack of fibronectin matrix deposition by these cells regardless of calreticulin expression level. Vinculin abundance did not appreciably increase upon calreticulin induction and the level of active c-Src, a regulatory kinase of focal contacts, was found to be abundant and unregulated by calreticulin induction in these cells. The inability to form stable focal contacts and to commence fibronectin fibrillogenesis due to high c-Src activity may be responsible for the poor adhesive phenotype of HEK 293 cells. Thus, we show here that HEK293 cells are not suitable for microscopical studies of cell-substratum adhesions, but are best suited for biochemical studies.

A novel Phex mutation with defective glycosylation causes hypophosphatemia and rickets in mice

N-ethyl-N-nitrosourea (ENU) mutagenesis is a phenotype-driven approach with potential to assign function to every locus in the mouse genome. In this article, we describe a new mutation, Pug, as a mouse model for X-linked hypophosphatemic rickets (XLH) in human. Mice carrying the Pug mutation exhibit abnormal phenotypes including growth retardation, hypophosphatemia and decreased bone mineral density (BMD). The new mutation was mapped to X-chromosome between 65.4 cM and 66.6 cM, where Phex gene resides. Sequence analysis revealed a unique T-to-C transition mutation resulting in Phe-to-Ser substitution at amino acid 80 of PHEX protein. In vitro studies of Pug mutation demonstrated that PHEX(pug) was incompletely glycosylated and sequestrated in the endoplasmic reticulum region of cell, whereas wild-type PHEX could be fully glycosylated and transported to the plasma membrane to exert its function as an endopeptidase. Taken together, the Pug mutant directly confirms the role of Phex in phosphate homeostasis and normal skeletal development and may serves as a new disease model of human hypophosphatemic rickets.

Pattern expression of glycan residues in AZT-treated K562 cells analyzed by lectin cytochemistry

The present paper shows that human chronic myeloid (K562) cells exposed 3 h to 20 microM 3'-azido-3'-deoxythymidine (AZT) exhibit marked variations of the oligosaccharide moiety of glycoconjugates. These changes were analyzed by confocal fluorescence microscopy, upon incubation of control and AZT-treated cells with biotin-lectin conjugates to visualize cell surface glycans or total glycans after cells permeabilization. In addition, cell fluorescence distribution of the biotinylated lectins, localized with streptavidin conjugates labeled with Alexa Fluor 488, was analyzed by flow cytometry. The results obtained show significant variations on the expression/distribution of membrane surface glycans as detected by both WGA and SNA, two lectins that recognize primarily cellular internal membrane glycolipids. A further interesting result was the significant increase of N-acetylglucosamine linked glycans localized either at the cell surface or intracellularly but only in K562 cells exposed to AZT. On the whole, our data demonstrate that AZT alters both lipid and N-linked glycosylations thus confirming previous observations, from our laboratory and from other Authors, that the drug impair the nucleotide-sugar import in the Golgi's lumen. AZT does also alter the O-linked glycosylations that occur in the Golgi complex since these reactions require the incorporation of sialic acid, GlcNAc and GalNAc all of which are sensitive to the drug.

Retention of Mutant Low Density Lipoprotein Receptor in Endoplasmic Reticulum (ER) Leads to ER Stress

Familial hypercholesterolemia is an autosomal dominant disease caused by mutations in the gene encoding the low density lipoprotein receptor (LDLR). More than 50% of these mutations lead to receptor proteins that are completely or partly retained in the endoplasmic reticulum (ER). The mechanisms involved in the intracellular processing and retention of mutant LDLR are poorly understood. In the present study we show that the G544V mutant LDLR associates with the chaperones Grp78, Grp94, ERp72, and calnexin in the ER of transfected Chinese hamster ovary cells. Retention of the mutant LDLR was shown to cause ER stress and activation of the unfolded protein response. We observed a marked increase in the activity of two ER stress sensors, IRE1 and PERK. These results show that retention of mutant LDLR in ER induces cellular responses, which might be important for the clinical outcome of familial hypercholesterolemia.

Molecular cloning and functional analysis of a novel Cx43 partner protein CIP150

We identified and cloned a novel gene encoding a partner protein, CIP150, of connexin 43 (Cx43). CIP150 associates with Cx43 through its carboxyl terminal domain. Conversely, a region consisting of 16 amino acids in the juxtamembrane region (amino acids 227-242) in the carboxyl terminal tail of Cx43 was identified to be responsible for the association. A variant of Cx43 lacking this region was expressed only in a nonphosphorylated form and appeared to lose the capacity to localize to the region of cell-cell contact and dye transfer activity. When the expression of CIP150 was suppressed using small interfering RNA, the localization to the plasma membrane as well as dye transfer activity of Cx43 was significantly reduced. These results suggest that the newly identified domain is essential for the proper phosphorylation and localization of Cx43, and CIP150 is a novel partner protein targeting this domain.

Cytosolic phospholipase A2-alpha mediates endothelial cell proliferation and is inactivated by association with the Golgi apparatus

Arachidonic acid and its metabolites are implicated in regulating endothelial cell proliferation. Cytosolic phospholipase A2-alpha (cPLA2alpha) is responsible for receptor-mediated arachidonic acid evolution. We tested the hypothesis that cPLA2alpha activity is linked to endothelial cell proliferation. The specific cPLA2alpha inhibitor, pyrrolidine-1, inhibited umbilical vein endothelial cell (HUVEC) proliferation in a dose-dependent manner. Exogenous arachidonic acid addition reversed this inhibitory effect. Inhibition of sPLA2 did not affect HUVEC proliferation. The levels of cPLA2alpha did not differ between subconfluent and confluent cultures of cells. However, using fluorescence microscopy we observed a novel, confluence-dependent redistribution of cPLA2alpha to the distal Golgi apparatus in HUVECs. Association of cPLA2alpha with the Golgi was linked to the proliferative status of HUVECs. When associated with the Golgi apparatus, cPLA2alpha activity was seen to be 87% inhibited. Relocation of cPLA2alpha to the cytoplasm and nucleus, and cPLA2alpha enzyme activity were required for cell cycle entry upon mechanical wounding of confluent monolayers. Thus, cPLA2alpha activity and function in controlling endothelial cell proliferation is regulated by reversible association with the Golgi apparatus.

Are transforming properties of the bovine papillomavirus E5 protein shared by E5 from high-risk human papillomavirus type 16?

The E5 proteins of bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 16 (HPV-16) are small (44-83 amino acids), hydrophobic polypeptides that localize to membranes of the Golgi apparatus and endoplasmic reticulum, respectively. While the oncogenic properties of BPV-1 E5 have been characterized in detail, less is known about HPV-16 E5 due to its low expression in mammalian cells. Using codon-optimized HPV-16 E5 DNA, we have generated stable fibroblast cell lines that express equivalent levels of epitope-tagged BPV-1 and HPV-16 E5 proteins. In contrast to BPV-1 E5, HPV-16 E5 does not activate growth factor receptors, phosphoinositide 3-kinase or c-Src, and fails to induce focus formation, although it does promote anchorage-independent growth in soft agar. These variant activities are apparently unrelated to differences in intracellular localization of the E5 proteins since retargeting HPV-16 E5 to the Golgi apparatus does not induce focus formation.

S-nitrosoprotein formation and localization in endothelial cells

Protein S-nitrosation represents a recently described form of post-translational modification that is rapid and reversible. However, the analysis of protein S-nitrosation in situ has been difficult because of the absence of specific probes and the instability of cellular protein S-nitrosothiols. We developed a rapid and specific method for detecting endothelial S-nitrosoproteins patterned after the biotin switch method that involves thiol alkylation followed by reductive generation of thiols from S-nitrosothiols, which are then labeled with either a biotin- or Texas red-derivative of methanethiosulfonate. When we used this methodology, we found that S-nitrosated proteins can form within endothelial cells from an exogenous S-nitrosothiol donor or from endogenous production of NO by endothelial NO synthase. When we used confocal microscopy, we found that these S-nitrosoproteins exist mainly in the mitochondria and peri-mitochondrial compartment, and that their half-life is approximately 1 h. Cellular S-nitrosated protein abundance changed as expected, with changes in activity of NO synthase, and with impairment of mitochondrial function and scavenging of peroxynitrite. We used a proteomic approach involving two-dimensional gel electrophoresis and mass spectrometry, and found that a limited number of S-nitrosoproteins exist in endothelial cells (S-nitrosoproteome) and identified GAPDH, vimentin, beta-galactosidase, peroxiredoxin 1, beta-actin, and ubiquitin-conjugating enzyme E2 among them. The most abundant S-nitrosated protein in the resting endothelial cell is GAPDH, suggesting a regulatory function for NO in glycolysis. These data offer methods and insights into identifying the protein targets of S-nitrosation reactions and their potential role in cell function and phenotype.

The Sec14 Homology Domain Regulates the Cellular Distribution and Transforming Activity of the Rho-specific Guanine Nucleotide Exchange Factor Dbs

Dbs is a Rho-specific guanine nucleotide exchange factor that was identified in a screen for proteins whose overexpression cause deregulated growth in murine fibroblasts. Dbs contains multiple recognizable motifs including a centrally located Rho-specific guanine nucleotide exchange factor domain, a COOH-terminal Src homology 3 domain, two spectrin-like repeats, and a recently identified NH(2)-terminal Sec14 homology domain. The transforming potential of Dbs is substantially activated by the removal of inhibitory sequences that lie outside of the core catalytic sequences, and in this current study we mapped this inhibition to the Sec14 domain. Surprisingly removal of the NH(2) terminus did not alter the catalytic activity of Dbs in vivo but rather altered its subcellular distribution. Whereas full-length Dbs was distributed primarily in a perinuclear structure that coincides with a marker for the Golgi apparatus, removal of the Sec14 domain was associated with translocation of Dbs to the cell periphery where it accumulated within membrane ruffles and lamellipodia. However, translocation of Dbs and the concomitant changes in the actin cytoskeleton were not sufficient to fully activate Dbs transformation. The Sec14 domain also forms intramolecular contacts with the pleckstrin homology domain, and these contacts must also be relieved to achieve full transforming activity. Collectively these observations suggest that the Sec14 domain regulates Dbs transformation through at least two distinct mechanisms, neither of which appears to directly influence the in vivo exchange activity of the protein.

Nudel functions in membrane traffic mainly through association with Lis1 and cytoplasmic dynein

Nudel and Lis1 appear to regulate cytoplasmic dynein in neuronal migration and mitosis through direct interactions. However, whether or not they regulate other functions of dynein remains elusive. Herein, overexpression of a Nudel mutant defective in association with either Lis1 or dynein heavy chain is shown to cause dispersions of membranous organelles whose trafficking depends on dynein. In contrast, the wild-type Nudel and the double mutant that binds to neither protein are much less effective. Time-lapse microscopy for lysosomes reveals significant reduction in both frequencies and velocities of their minus end–directed motions in cells expressing the dynein-binding defective mutant, whereas neither the durations of movement nor the plus end–directed motility is considerably altered. Moreover, silencing Nudel expression by RNA interference results in Golgi apparatus fragmentation and cell death. Together, it is concluded that Nudel is critical for dynein motor activity in membrane transport and possibly other cellular activities through interactions with both Lis1 and dynein heavy chain.

Association of cPLA2-alpha and COX-1 with the Golgi apparatus of A549 human lung epithelial cells

Cytosolic phospholipase A2-alpha (cPLA2-alpha) is an 85 kDa, Ca2+-sensitive enzyme involved in receptor-mediated prostaglandin synthesis. In airway epithelial cells, the release of prostaglandins is crucial in regulating the inflammatory response. Although prostaglandin release has been studied in various epithelial cell models, the subcellular location of cPLA2-alpha in these cells is unknown. Using high-resolution confocal microscopy of the human A549 lung epithelial cell line, we show that cPLA2-alpha relocates from the cytosol and nuclei to a juxtanuclear region following stimulation with the Ca2+ ionophore A23187. Double staining with rhodamine-conjugated wheat germ agglutinin confirmed this region to be the Golgi apparatus. Markers specific for Golgi subcompartments revealed that cPLA2-alpha is predominantly located at the trans-Golgi stack and the trans-Golgi network following elevation of cytosolic Ca2+. Furthermore, treatment of cells with the Golgi-disrupting agent brefeldin A caused a redistribution of cPLA2-alpha, confirming that cPLA2-alpha associates with Golgi-derived membranes. Finally, a specific co-localization of cPLA2-alpha with cyclooxygenase-1 but not cyclooxygenase-2 was evident at the Golgi apparatus. These results, combined with recent data on the role of PLA2 activity in maintaining Golgi structure and function, suggest that Golgi localization of cPLA2-alpha may be involved in membrane trafficking in epithelial cells.

The C-terminal domain, but not the interchain disulphide, is required for the activity and intracellular trafficking of aminopeptidase A

Mammalian aminopeptidase A (APA; glutamyl aminopeptidase; EC 3.4.11.7) is a type II membrane-spanning protein consisting of a short N-terminal cytosolic domain, a single transmembrane domain and a large extracellular C-terminal domain containing the active site. The extracellular domain consists of a 107 kDa domain, containing the zinc-binding motif and all the residues involved in catalysis, separated by a protease-susceptible hinge region from the 45 kDA C-terminal domain of unknown function. To investigate the role of the 45 kDa domain, a construct of murine APA (G594Delta) lacking this C-terminal domain was expressed in COS-1 cells. This truncated form of APA, although expressed, lacked enzymic activity and failed to reach the cell surface. Confocal immunofluorescence microscopy revealed that G594Delta co-localized with the lectin concanavalin A and had a similar staining pattern as protein disulphide-isomerase, indicating that it was retained in the endoplasmic reticulum. Thus the C-terminal 45 kDa domain appears to be acting like a pro-domain and seems to be required for the correct folding and trafficking of APA. In contrast, mutation of cysteine-43 to serine, which is involved in the disulphide-linkage of the APA homodimer, did not affect the enzymic activity, cellular location or rate of trafficking through the secretory pathway of APA.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Mutant rab8 Impairs docking and fusion of rhodopsin-bearing post-Golgi membranes and causes cell death of transgenic Xenopus rods

Rab8 is a GTPase involved in membrane trafficking. In photoreceptor cells, rab8 is proposed to participate in the late stages of delivery of rhodopsin-containing post-Golgi membranes to the plasma membrane near the base of the connecting cilium. To test the function of rab8 in vivo, we generated transgenic Xenopus laevis expressing wild-type, constitutively active (Q67L), and dominant negative (T22N) forms of canine rab8 in their rod photoreceptors as green fluorescent protein (GFP) fusion proteins. Wild-type and constitutively active GFP-rab8 proteins were primarily associated with Golgi and post-Golgi membranes, whereas the dominant negative protein was primarily cytoplasmic. Expression of wild-type GFP-rab8 had minimal effects on cell survival and intracellular structures. In contrast, GFP-rab8T22N caused rapid retinal degeneration. In surviving peripheral rods, tubulo-vesicular structures accumulated at the base of the connecting cilium. Expression of GFP-rab8Q67L induced a slower retinal degeneration in some tadpoles. Transgene effects were transmitted to F1 offspring. Expression of the GFP-rab8 fusion proteins appears to decrease the levels of endogenous rab8 protein. Our results demonstrate a role for rab8 in docking of post-Golgi membranes in rods, and constitute the first report of a transgenic X. laevis model of retinal degenerative disease.

Apaf-1 localization is modulated indirectly by Bcl-2 expression

Apoptotic protease activating factor-1 (Apaf-1) is an adaptor molecule essential for caspase-9 activation. Subcellular analysis of Apaf-1 in NIH-3T3 fibroblasts and the immature murine B cell lymphoma WEHI-231 indicates that Apaf-1 is localized in the Golgi apparatus and cytoplasm. Bcl-2 overexpression in WEHI-231 cells disrupts Apaf-1 localization in Golgi, causing a perinuclear Apaf-1 redistribution. Bcl-2 overexpression in NIH-3T3 fibroblasts however does not cause similar Apaf-1 redistribution, suggesting that cell type factors are involved in the redistribution process. The ability of Bcl-2 to modify Apaf-1 subcellular localization is not explained by direct interaction between Apaf-1 and Bcl-2. These data may help to clarify the anti-apoptotic Bcl-2 function.

Chick ciliary neurotrophic factor is secreted via a nonclassical pathway

In contrast to mammalian ciliary neurotrophic factors (CNTFs), chick CNTF is secreted, although it lacks an N-terminal signal. We determined that a 52 aa region of chick CNTF containing an internal hydrophobic domain could direct secretion of rat CNTF. Using a stable cell line that overexpressed chick CNTF, we found that chick CNTF immunoreactivity was punctate throughout the cytosol. Cellular fractionation confirmed chick CNTF to be protected by vesicles. Chick CNTF did not colocalize with fibronectin, calreticulin, wheat germ agglutinin binding sites, or with transferrin receptor. The distribution of chick CNTF was altered neither by brefeldin A nor by chloroquine treatment. Although the punctate pattern of chick CNTF immunoreactivity was not due to reuptake, chick CNTF could be found in a cellular compartment labeled after a brief incubation with dextran microbeads. When synthesized in vitro, chick CNTF did not translocate into microsomes. We conclude that chick CNTF is secreted via a nonclassical pathway.

Cloning of ACP33 as a novel intracellular ligand of CD4

CD4 recruitment to T cell receptor (TCR)-peptide-major histocompatibility class II complexes is required for stabilization of low affinity antigen recognition by T lymphocytes. The cytoplasmic portion of CD4 is thought to amplify TCR-initiated signal transduction via its association with the protein tyrosine kinase p56(lck). Here we describe a novel functional determinant in the cytosolic tail of CD4 that inhibits TCR-induced T cell activation. Deletion of two conserved hydrophobic amino acids from the CD4 carboxyl terminus resulted in a pronounced enhancement of CD4-mediated T cell costimulation. This effect was observed in the presence or absence of p56(lck), implying involvement of alternative cytosolic ligands of CD4. A two-hybrid screen with the intracellular portion of CD4 identified a previously unknown 33-kDa protein, ACP33 (acidic cluster protein 33), as a novel intracellular binding partner of CD4. Since interaction with ACP33 is abolished by deletion of the hydrophobic CD4 C-terminal amino acids mediating repression of T cell activation, we propose that ACP33 modulates the stimulatory activity of CD4. Furthermore, we demonstrate that interaction with CD4 is mediated by the noncatalytic alpha/beta hydrolase fold domain of ACP33. This suggests a previously unrecognized function for alpha/beta hydrolase fold domains as a peptide binding module mediating protein-protein interactions.

S-adenosylmethionine reverses ilimaquinone's vesiculation of the Golgi apparatus: a fluorescence study on the cellular interactions of ilimaquinone

The marine sponge metabolite ilimaquinone has a wide range of biological activities, including vesiculation of the Golgi apparatus and interference with intracellular protein trafficking. Some of these activities may arise from ilimaquinone's influence on the activated methyl cycle. To visualize the morphological effects of ilimaquinone on the Golgi apparatus, NRK (normal rat kidney) cells were labeled with fluorescent wheat germ agglutinin and treated with ilimaquinone in the presence and absence of the methylating agent S-adenosylmethionine (SAMe). While ilimaquinone alone fragments the Golgi apparatus, the organelle remains intact when SAMe is included in the incubation mixture. This observation supports ilimaquinone's interaction with methylation enzymes as the cause of Golgi vesiculation. The examination of a fluorescently labeled ilimaquinone analogue in NRK cells suggests that the cellular interactions of ilimaquinone are not localized to the Golgi apparatus.

Identification of an outer segment targeting signal in the COOH terminus of rhodopsin using transgenic Xenopus laevis

Mislocalization of the photopigment rhodopsin may be involved in the pathology of certain inherited retinal degenerative diseases. Here, we have elucidated rhodopsin's targeting signal which is responsible for its polarized distribution to the rod outer segment (ROS). Various green fluorescent protein (GFP)/rhodopsin COOH-terminal fusion proteins were expressed specifically in the major red rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The fusion proteins were targeted to membranes via lipid modifications (palmitoylation and myristoylation) as opposed to membrane spanning domains. Membrane association was found to be necessary but not sufficient for efficient ROS localization. A GFP fusion protein containing only the cytoplasmic COOH-terminal 44 amino acids of Xenopus rhodopsin localized exclusively to ROS membranes. Chimeras between rhodopsin and alpha adrenergic receptor COOH-terminal sequences further refined rhodopsin's ROS localization signal to its distal eight amino acids. Mutations/deletions of this region resulted in partial delocalization of the fusion proteins to rod inner segment (RIS) membranes. The targeting and transport of endogenous wild-type rhodopsin was unaffected by the presence of mislocalized GFP fusion proteins.

Distinct Influences of Carboxyl Terminal Segment Structure on Function in the Two Isoforms of Prostaglandin H Synthase

The cyclooxygenase activity of the two prostaglandin H synthase (PGHS) isoforms, PGHS-1 and -2, is a major control element in prostanoid biosynthesis. The two PGHS isoforms have 60% amino acid identity, with prominent differences near the C-terminus, where PGHS-2 has an additional 18-residue insert. Some mutations of the C-terminal residue in PGHS-1 and -2 have been found to disrupt catalytic activity and/or intracellular targeting of the proteins, but the relationship between C-terminal structure and function in the two isoforms has been poorly defined. Crystallographic data indicate the PGHS-1 and -2 C-termini are positioned to interact with the endoplasmic reticulum (ER) membrane, although the C-terminal segment structure was not resolved for either isoform. We constructed a series of C-terminal substitution, deletion, and insertion mutants of human PGHS-1 and -2 and evaluated the effects on cyclooxygenase activity and intracellular targeting in transfected COS-1 cells expressing the recombinant proteins. PGHS-1 cyclooxygenase activity was strongly disrupted by C-terminal substitutions and deletions, but not by elongation of the C-terminal segment, even when the ultimate residue was altered. Similar alterations to PGHS-2 had markedly less effect on cyclooxygenase activity. The results indicate that the functioning of the longer C-terminal segment in PGHS-2 is distinctly more tolerant of structural change than the shorter PGHS-1 C-terminal segment. C-Terminal substitutions or deletions did not change the subcellular localization of either isoform, even at short times after transfection, indicating that neither C-terminal segment contains indispensable intracellular targeting signals.

Targeted deletion of Minpp1 provides new insight into the activity of multiple inositol polyphosphate phosphatase in vivo

Multiple inositol polyphosphate phosphatase (Minpp1) metabolizes inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol hexakisphosphate (InsP(6)) with high affinity in vitro. However, Minpp1 is compartmentalized in the endoplasmic reticulum (ER) lumen, where access of enzyme to these predominantly cytosolic substrates in vivo has not previously been demonstrated. To gain insight into the physiological activity of Minpp1, Minpp1-deficient mice were generated by homologous recombination. Tissue extracts from Minpp1-deficient mice lacked detectable Minpp1 mRNA expression and Minpp1 enzyme activity. Unexpectedly, Minpp1-deficient mice were viable, fertile, and without obvious defects. Although Minpp1 expression is upregulated during chondrocyte hypertrophy, normal chondrocyte differentiation and bone development were observed in Minpp1-deficient mice. Biochemical analyses demonstrate that InsP(5) and InsP(6) are in vivo substrates for ER-based Minpp1, as levels of these polyphosphates in Minpp1-deficient embryonic fibroblasts were 30 to 45% higher than in wild-type cells. This increase was reversed by reintroducing exogenous Minpp1 into the ER. Thus, ER-based Minpp1 plays a significant role in the maintenance of steady-state levels of InsP(5) and InsP(6). These polyphosphates could be reduced below their natural levels by aberrant expression in the cytosol of a truncated Minpp1 lacking its ER-targeting N terminus. This was accompanied by slowed cellular proliferation, indicating that maintenance of cellular InsP(5) and InsP(6) is essential to normal cell growth. Yet, depletion of cellular inositol polyphosphates during erythropoiesis emerges as an additional physiological activity of Minpp1; loss of this enzyme activity in erythrocytes from Minpp1-deficient mice was accompanied by upregulation of a novel, substitutive inositol polyphosphate phosphatase.

Identification of the isoforms of Ca(2+)/Calmodulin-dependent protein kinase II in rat astrocytes and their subcellular localization

Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II) occurs in astrocytes as well as in neurons in brain. We have reported that CaM kinase II is involved in the regulation of cytoskeletal proteins and gene expression in astrocytes. In this study, we identified all isoforms of CaM kinase II in astrocytes and examined their subcellular localization. When we amplified the isoforms of four subunits by RT-PCR followed by the "nested" PCR, totally 10 isoforms were obtained. Immunoblot analyses with five types of antibodies against CaM kinase II indicated that the most abundant isoform was delta2. Immunostaining suggested that the delta2 isoform was localized predominantly at the Golgi apparatus. The localization of the delta2 isoform at the Golgi apparatus was also observed in NG108-15 cells. We overexpressed all isoforms that contained the nuclear localization signal to examine their nuclear targeting in NG108-15 cells. In contrast to the alphaB and delta3 isoforms that entered the nucleus, as reported, the gammaA isoform was excluded from the nucleus in the transfected NG108-15 cells. These results suggest that the 15-amino acid insertion following the nuclear localization signal inhibits the nuclear targeting of the gammaA isoform.

Endogenously expressed apolipoprotein E has different effects on cell lipid metabolism as compared to exogenous apolipoprotein E carried on triglyceride-rich particles

Apolipoprotein E (apoE) on model triglyceride-rich particles (TGRP) increases triglyceride (TG) utilization and cholesteryl ester (CE) hydrolysis, independent of its effect on enhancing particle uptake. We questioned whether, under physiological concentrations, endogenously expressed apoE has similar effects on cellular lipid metabolism as compared to exogenous apoE. J774 macrophages, which do not express apoE, were engineered to express endogenous apoE by transfection of human apoE3 cDNA expression constructs (E(+)) or control vectors (E(-)) into the cells. To compare the effects of exogenous apoE and endogenous apoE on TGRP uptake, cells were incubated with or without apoE associated with (3)H-cholesteryl ether-labeled TGRP. Exogenous apoE enhanced TGRP uptake in both E(-) and E(+) cells. E(-) cells displayed significantly higher TGRP uptake than E(+) cells. Sodium chlorate, which inhibits cell proteoglycan synthesis, markedly diminished differences in TGRP uptake between E(-) and E(+) cells, suggesting that endogenous apoE-proteoglycan interaction contributes to differences in uptake between the two cell types. Particle uptake by the LDL receptor, by the LDL receptor related protein, or by scavenger receptors were similar between E(-) and E(+) cells indicating that endogenous apoE expression does not have a general effect on endocytic pathways. Exogenous apoE carried on TGRP stimulated TG utilization and CE hydrolysis in both cell types. However, TG utilization and CE hydrolysis were not affected by endogenous apoE expression. In conclusion, macrophage expression of apoE has very different effects on TGRP metabolism than exogenously supplied apoE. The fluorescence microscopy results in this study showing that exogenous apoE and endogenous apoE were confined in separate cellular compartments support the hypothesis that these differences resulted from distinct intracellular trafficking pathways followed by exogenous apoE bound to TGRP as compared to endogenous cell-expressed apoE.

Differential localization and regulation of death and decoy receptors for TNF-related apoptosis-inducing ligand (TRAIL) in human melanoma cells

Induction of apoptosis in cells by TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF family, is believed to be regulated by expression of two death-inducing and two inhibitory (decoy) receptors on the cell surface. In previous studies we found no correlation between expression of decoy receptors and susceptibility of human melanoma cells to TRAIL-induced apoptosis. In view of this, we studied the localization of the receptors in melanoma cells by confocal microscopy to better understand their function. We show that the death receptors TRAIL-R1 and R2 are located in the trans-Golgi network, whereas the inhibitory receptors TRAIL-R3 and -R4 are located in the nucleus. After exposure to TRAIL, TRAIL-R1 and -R2 are internalized into endosomes, whereas TRAIL-R3 and -R4 undergo relocation from the nucleus to the cytoplasm and cell membranes. This movement of decoy receptors was dependent on signals from TRAIL-R1 and -R2, as shown by blocking experiments with Abs to TRAIL-R1 and -R2. The location of TRAIL-R1, -R3, and -R4 in melanoma cells transfected with cDNA for these receptors was similar to that in nontransfected cells. Transfection of TRAIL-R3 and -R4 increased resistance of the melanoma lines to TRAIL-induced apoptosis even in melanoma lines that naturally expressed these receptors. These results indicate that abnormalities in "decoy" receptor location or function may contribute to sensitivity of melanoma to TRAIL-induced apoptosis and suggest that further studies are needed on the functional significance of their nuclear location and TRAIL-induced movement within cells.

Intracellular and extracellular leukemia inhibitory factor proteins have different cellular activities that are mediated by distinct protein motifs

Although many growth factors and cytokines have been shown to be localized within the cell and nucleus, the mechanism by which these molecules elicit a biological response is not well understood. The cytokine leukemia inhibitory factor (LIF) provides a tractable experimental system to investigate this problem, because translation of alternatively spliced transcripts results in the production of differentially localized LIF proteins, one secreted from the cell and acting via cell surface receptors and the other localized within the cell. We have used overexpression analysis to demonstrate that extracellular and intracellular LIF proteins can have distinct cellular activities. Intracellular LIF protein is localized to both nucleus and cytoplasm and when overexpressed induces apoptosis that is inhibited by CrmA but not Bcl-2 expression. Mutational analysis revealed that the intracellular activity was independent of receptor interaction and activation and reliant on a conserved leucine-rich motif that was not required for activation of cell surface receptors by extracellular protein. This provides the first report of alternate intracellular and extracellular cytokine activities that result from differential cellular localization of the protein and are mediated by spatially distinct motifs.

Role of the Gag matrix domain in targeting human immunodeficiency virus type 1 assembly

Human immunodeficiency virus type 1 (HIV-1) particle formation and the subsequent initiation of protease-mediated maturation occur predominantly on the plasma membrane. However, the mechanism by which HIV-1 assembly is targeted specifically to the plasma membrane versus intracellular membranes is largely unknown. Previously, we observed that mutations between residues 84 and 88 of the matrix (MA) domain of HIV-1 Gag cause a retargeting of virus particle formation to an intracellular site. In this study, we demonstrate that the mutant virus assembly occurs in the Golgi or in post-Golgi vesicles. These particles undergo core condensation in a protease-dependent manner, indicating that virus maturation can occur not only on the plasma membrane but also in the Golgi or post-Golgi vesicles. The intracellular assembly of mutant particles is dependent on Gag myristylation but is not influenced by p6(Gag) or envelope glycoprotein expression. Previous characterization of viral revertants suggested a functional relationship between the highly basic domain of MA (amino acids 17 to 31) and residues 84 to 88. We now demonstrate that mutations in the highly basic domain also retarget virus particle formation to the Golgi or post-Golgi vesicles. Although the basic domain has been implicated in Gag membrane binding, no correlation was observed between the impact of mutations on membrane binding and Gag targeting, indicating that these two functions of MA are genetically separable. Plasma membrane targeting of Gag proteins with mutations in either the basic domain or between residues 84 and 88 was rescued by coexpression with wild-type Gag; however, the two groups of MA mutants could not rescue each other. We propose that the highly basic domain of MA contains a major determinant of HIV-1 Gag plasma membrane targeting and that mutations between residues 84 and 88 disrupt plasma membrane targeting through an effect on the basic domain.

Human follicle stimulating hormone receptor variants lacking transmembrane domains display altered post-translational conformations

Variant splicing of gonadotropin receptor mRNA commonly occurs, however expression of receptor protein variants and their trafficking has yet to be studied in detail. To determine receptor variant trafficking and intracellular processing in mammalian cells, the intracellular fate of intentionally truncated variants of human follicle stimulating hormone receptor (hFSH-R) expressed in CHO cells was examined. Monoclonal antibodies (mAbs) were made against the hFSH-R's extracellular domain (ECD) expressed in insect cells. Four mAbs 106.156, 106.290, 106.318, and 106.263 were chosen as probes. Epitope mapping using synthetic peptides, and truncated hFSH-R variants revealed that mAb 106.156 bound to ECD residues 183-220, while mAbs 106.318, 106.290, 106.263 bound ECD residues 300-331. Immunofluorescence microscopy showed that mAbs 106.318 and 106.156 stained the surface of fixed, intact CHO cells expressing wild type hFSH-R. However, following cell permeabilization all four antibodies stained hFSH-R in Golgi and endoplasmic reticulum. Permeabilized cells expressing truncated variants ECD213 and ECD254 showed staining accumulated in the endoplasmic reticulum/nuclear envelope continuum. ECD335/His was found to accumulate in extended endoplasmic reticulum (ER). The ER location of ECD335/His was confirmed by double labeling experiments with concanavalin A and ECD mAb. Glycosidase digestion followed by Western blot analysis show ECD213 and ECD335/His to be glycosylated, but not ECD254. Both glycosylated truncated hFSH-R variants were sensitive to peptide-N-glycanase F and endoglycosidase H but insensitive to neuraminidase indicating that these variants possess high mannose type oligosaccharides. Thus truncated hFSH-R variants do not reach the medial or trans Golgi where high mannose oligosaccharides are trimmed and sialic acid is added. These data suggest that the conformation the ECD of the wild type receptor is different from the ECD alone expressed in the endoplasmic reticulum. This information suggests that the ECD serves two distinct roles; the first is to bind FSH and the other is likely to contact the endodomain of the receptor, which presumably leads to activation of the endodomain for signal transduction.

Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms. Identification of a new CCTbeta splice variant

CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.

B-50/GAP-43 potentiates cytoskeletal reorganization in raft domains

B-50 (GAP-43) is a neural, membrane-associated protein that has been implicated in neurite outgrowth and guidance. Following stable transfection of Rat1 fibroblasts with B-50 cDNA we observed a dispersed distribution of B-50 immunoreactivity in flattened resting cells. In contrast, motile cells exhibited high concentrations of B-50 at the leading edge of ruffling membranes, coinciding with actin polymerization. Time-lapse studies on Rat1 fibroblasts transiently transfected with B-50/EGFP revealed that large vesicles originated from the ruffling membranes. These large vesicles (pinocytes) were found positive for Thy-1, a GPI-anchored protein, but negative for rab-5, an early endosome marker. In primary hippocampal neurons B-50 also colocalized completely with the raft marker Thy-1. Antibody-mediated cross-linking of Thy-1 in hippocampal neurons resulted in a redistribution of the intracellular protein B-50 to Thy-1-immunopositive membrane patches, whereas syntaxin was mainly excluded from the patches, showing that B-50 is associated with rafts. Academic Press.

Localization of ADAM10 and Notch receptors in bone

In Drosophila melanogaster, the role of the metallodisintegrin, Kuzbanian (kuz), is thought to involve activation of the Drosophila Notch receptor that plays a role in cell-fate determination during neurogenesis and myoblast differentiation. To understand the possible function(s) of a-disintegrin and metalloproteinase (ADAM10), the mammalian ortholog of kuz, in the skeleton, we studied its expression as well as the messenger RNA (mRNA) encoding one candidate substrate, the mammalian Notch2 receptor in bone, bone cells, and cartilage. In sections of neonatal rat tibiae, ADAM10 is expressed in specific regions of articular cartilage and metaphyseal bone. Expression of ADAM10 in articular cartilage occurs predominantly in superficial chondrocytes and becomes more sporadic with increasing distance from the articular surface. In bone, ADAM10 is expressed by periosteal cells, osteoblasts, and osteocytes at locations of active bone formation. Osteoclasts did not express ADAM10. Notch2 mRNA expression was not detectable in superficial chondrocytes. However it colocalized at all sites of ADAM10 expression in bone cells. In vitro, both primary human osteoblasts and osteoblast cell lines expressed a single 4.5 kb and 7.5 kb transcript of ADAM10 and the Notch2 receptor homolog, respectively. Subcellular localization of the ADAM10 protein in MG-63 cells was determined using immunofluorescent techniques. These observations showed clearly that the ADAM10 protein was expressed in the trans-Golgi network and on the plasma membrane. Western blot analysis of fractionated cells showed that, in the plasma membrane fraction, the previously characterized 58 kDa and 56 kDa isoforms were present, whereas, in the trans-Golgi network, the ADAM10 protein was present in several additional bands, possibly indicative of further interdomain processing of the ADAM10 protein. The metallodisintegrins (ADAMs) have several putative functions, including modulation of cell adhesion, membrane-associated proteolysis, and cell-cell signaling. These observations suggest that, in bone but not cartilage, ADAM10 has catalytic activity within the transGolgi network and may play a role in the activation of Notch receptor homologs. This implicates ADAM10 in cell-fate determination of osteoblast progenitor cells, possibly during skeletal development and normal bone remodeling. Plasma-membrane-associated ADAM10 may confer alternative functions.

Truncation releases olfactory receptors from the endoplasmic reticulum of heterologous cells

Olfactory receptors are difficult to express functionally in heterologous cells. We found that olfactory receptors traffic poorly to the plasma membrane even in cells with neuronal phenotypes, including cell lines derived from the olfactory epithelium. Other than mature olfactory receptor neurons, few cells appear able to traffic olfactory receptors to the plasma membrane. In human embryonic kidney 293 cells and Xenopus fibroblasts, olfactory receptor immunoreactivity overlapped with a marker for the endoplasmic reticulum (ER) but not with markers for the Golgi apparatus or endosomes. Except for the ER, olfactory receptors were therefore absent from organelles normally involved in the plasma membrane trafficking of receptors. Olfactory receptors truncated prior to transmembrane domain VI were expressed in the plasma membrane, however. Co-expression of the missing C-terminal fragment with these truncated receptors prevented their expression in the plasma membrane. Intramolecular interactions between N- and C-terminal domains joined by the third cytoplasmic loop appear to be responsible for retention of olfactory receptors in the ER of heterologous cells. Our results are consistent with misfolding of the receptors but could also be explained by altered trafficking of the receptors.

Calreticulin affects focal contact-dependent but not close contact-dependent cell-substratum adhesion

We used two cell lines expressing fast (RPEfast) and slow (RPEslow) attachment kinetics to investigate mechanisms of cell-substratum adhesion. We show that the abundance of a cytoskeletal protein, vinculin, is dramatically decreased in RPEfast cells. This coincides with the diminished expression level of an endoplasmic reticulum chaperone, calreticulin. Both protein and mRNA levels for calreticulin and vinculin were decreased in RPEfast cells. After RPEfast cells were transfected with cDNA encoding calreticulin, both the expression of endoplasmic reticulum-resident calreticulin and cytoplasmic vinculin increased. The abundance of other adhesion-related proteins was not affected. RPEfast cells underexpressing calreticulin displayed a dramatic increase in the abundance of total cellular phosphotyrosine suggesting that the effects of calreticulin on cell adhesiveness may involve modulation of the activities of protein tyrosine kinases or phosphatases which may affect the stability of focal contacts. The calreticulin and vinculin underexpressing RPEfast cells lacked extensive focal contacts and adhered weakly but attached fast to the substratum. In contrast, the RPEslow cells that expressed calreticulin and vinculin abundantly developed numerous and prominent focal contacts slowly, but adhered strongly. Thus, while the calreticulin overexpressing RPEslow cells "grip" the substratum with focal contacts, calreticulin underexpressing RPEfast cells use close contacts to "stick" to it.

Cellular responses to excess phospholipid

Phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells, and its synthesis is controlled by the activity of CDP:phosphocholine cytidylyltransferase (CCT). Enforced CCT expression accelerated the rate of PtdCho synthesis. However, the amount of cellular PtdCho did not increase as a result of the turnover of both the choline and glycerol components of PtdCho. Metabolic labeling experiments demonstrated that cells compensated for elevated CCT activity by the degradation of PtdCho to glycerophosphocholine (GPC). Phospholipase D-mediated PtdCho hydrolysis and phosphocholine formation were unaffected. Most of the GPC produced in response to excess phospholipid production was secreted into the medium. Cells also degraded the excess membrane PtdCho to GPC when phospholipid formation was increased by exposure to exogenous lysophosphatidylcholine or lysophosphatidylethanolamine. The replacement of the acyl moiety at the 1-position of PtdCho with a non-hydrolyzable alkyl moiety prevented degradation to GPC. Accumulation of alkylacyl-PtdCho was associated with the inhibition of cell proliferation, demonstrating that alternative pathways of degradation will not substitute. GPC formation was blocked by bromoenol lactone, implicating the calcium-independent phospholipase A2 as a key participant in the response to excess phospholipid. Owing to the fact that PtdCho is biosynthetically converted to PtdEtn, excess PtdCho resulted in overproduction and exit of GPE as well as GPC. Thus, general membrane phospholipid homeostasis is achieved by a balance between the opposing activities of CCT and phospholipase A2.