Cell-surface expression of a membrane-anchored form of the human chorionic gonadotropin alpha subunit

Membrane-tethered proteins for basic research, imaging, and therapy

Secreted and intracellular proteins including antibodies, cytokines, major histocompatibility complex molecules, antigens, and enzymes can be redirected to and anchored on the surface of mammalian cells to reveal novel functions and properties such as reducing systemic toxicity, altering the in vivo distribution of drugs and extending the range of useful drugs, creating novel, specific signaling receptors and reshaping protein immunogenicity. The present review highlights progress in designing vectors to target and retain chimeric proteins on the surface of mammalian cells. Comparison of chimeric proteins indicates that selection of the proper cytoplasmic domain and introduction of oligiosaccharides near the cell surface can dramatically enhance surface expression, especially for single-chain antibodies. We also describe progress and limitations of employing surface-tethered proteins for preferential activation of prodrugs at cancer cells, imaging gene expression in living animals, performing high-throughput screening, selectively activating immune cells in tumors, producing new adhesion molecules, creating local immune privileged sites, limiting the distribution of soluble factors such as cytokines, and enhancing polypeptide immunogenicity. Surface-anchored chimeric proteins represent a rich source for developing new techniques and creating novel therapeutics.

A gene therapy approach for treating T-cell-mediated autoimmune diseases

Experimental autoimmune encephalomyelitis (EAE) is a demyelinating disease of the central nervous system (CNS) that serves as a model for multiple sclerosis (MS) in humans. In mice, EAE is mediated by Th1 type CD4(+) T cells specific for various myelin proteins which migrate from the periphery to the CNS. Removal or blocking of CD4(+) cells before or shortly after disease induction was shown to prevent disease onset and/or disease progression but also results in general immune suppression. Most treatment regimens for autoimmune diseases currently rely on general suppression of the T-cell compartment most commonly by steroids. In this paper, an experimental, gene therapy-based model is presented in which susceptible mice are made resistant to EAE induction by specifically down-regulating an autoreactive T-cell population. By using a retroviral gene transfer protocol, normal B cells were genetically modified to constitutively express the SJL-specific proteolipid (PLP) encephalitogenic determinant and then adoptively transferred into syngeneic hosts. To ensure appropriate presentation of the exogenous encephalitogenic peptide in association with MHC class II, the encephalitogenic sequence was fused to a lysosomal targeting sequence. Adoptive transfer of syngeneic B cells expressing the PLP encephalitogenic determinant into normal, naive, genetically susceptible mice induced PLP-specific unresponsiveness and completely protected the majority (62% and 83% using an intermediate and a high titer retroviral vector, respectively) of the animals from EAE induction. The remaining animals had a delayed disease onset and/or lower disease severity. All protected mice expressed the exogenous gene in the spleen as detected by reverse transcriptase-polymerase chain reaction.

Modification of a recombinant GPI-anchored metalloproteinase for secretion alters the protein glycosylation

The N-linked glycans of recombinant leishmanolysin (GP63) expressed as a glycosylphosphatidylinositol (GPI)-anchored membrane protein or modified for secretion in Chinese hamster ovary (CHO) cells were analyzed by fast atom bombardment-mass spectrometry (FAB-MS). The glycans isolated from both membrane and secreted protein were predominantly complex biantennary structures. However other aspects of the glycan profiles showed striking differences. The degree of sialylation of the membrane form was greatly reduced and the core fucosylation of biantennary structures was increased compared to the secreted form. Glycans isolated from membrane expressed protein also contained a higher proportion of lactosamine repeats. Residence times in the secretory pathway were similar for both secreted and membrane protein. Glycosylation differences may therefore be due to differences in protein conformation and accessibility to glycosyltransferases or glycosidases. These differences in glycosylation represent an important factor when considering modifying membrane expressed proteins for secreted production.

Identification of novel transmembrane gene sequence and its use for cell-surface targeting of beta subunit of human chorionic gonadotropin

We identified a 685-nucleotide gene fragment that codes for the transmembrane and cytoplasmic domains of glycoprotein of the LEP strain rabies virus and carried out experiments designed to express a novel fusion protein on the cell surface. The cDNA encoding the membrane anchor sequence was fused in the correct reading frame to the 3' end of the cDNA encoding the beta subunit of human chorionic gonadotropin (beta(h)CG), a secretory glycoprotein that is used as an antigen for a contraceptive vaccine being developed in our laboratory. The fusion gene cassette was placed under the control of a vaccinia virus early promoter and cloned in a host-restricted fowlpox viral vector. The recombinants, when used to infect mammalian cells that do not allow the replication of fowlpox virus, expressed the N-terminal 135 amino acid residues of beta(h)CG anchored in the cell membrane by the 75-amino acid C-terminal sequence derived from rabies virus glycoprotein. This hybrid protein is correctly processed post-translationally and transported efficiently to the plasma membrane of non-permissive cells such that the anchored beta(h)CG molecule retains the correctly folded native antigenic epitope(s).

The role of site-specific N-glycosylation in secretion of soluble forms of rabies virus glycoprotein

Rabies virus glycoprotein is important in the biology and pathogenesis of neurotropic rabies virus infection. This transmembrane glycoprotein is the only viral protein on the surface of virus particles, is the viral attachment protein that facilitates virus uptake by the infected cell, and is the target of the host humoral immune response to infection. The extracellular domain of this glycoprotein has N-glycosylation sequons at Asn37, Asn247, and Asn319. Appropriate glycosylation of these sequons is important in the expression of the glycoprotein. Soluble forms of rabies virus glycoprotein were constructed by insertion of a stop codon just external to the transmembrane domain. Using site-directed mutagenesis and expression in transfected eukaryotic cells, it was possible to compare the effects of site-specific glycosylation on the cell-surface expression and secretion of transmembrane and soluble forms, respectively, of the same glycoprotein. These studies yielded the surprising finding that although any of the three sequons permitted cell surface expression of full-length rabies virus glycoprotein, only the N-glycan at Asn319 permitted secretion of soluble rabies virus glycoprotein. Despite its biological and medical importance, it has not yet been possible to determine the crystal structure of the full-length transmembrane form of rabies virus glycoprotein which contains heterogeneous oligosaccharides. The current studies demonstrate that a soluble form of rabies virus glycoprotein containing only one sequon at Asn319 is efficiently secreted in the presence of the N-glycan processing inhibitor 1-deoxymannojirimycin. Thus, it is possible to purify a conformationally relevant form of rabies virus glycoprotein that contains only one N-glycan with a substantial reduction in its microheterogeneity. This form of the glycoprotein may be particularly useful for future studies aimed at elucidating the three-dimensional structure of this important glycoprotein.

Targeting vaccinia virus-expressed secretory beta subunit of human chorionic gonadotropin to the cell surface induces antibodies

We carried out experiments designed to study the effect of a protein's localization on its immunogenicity. A novel cell-surface protein was generated from a small, glycosylated secretory protein. The DNA sequence encoding the entire precursor of the human chorionic gonadotropin beta (beta hCG) subunit was fused in the correct reading frame to the DNA sequence encoding the transmembrane and cytoplasmic domains of vesicular stomatitis virus glycoprotein. This chimeric gene was introduced into the vaccinia virus genome to generate a recombinant virus. The recombinant virus, when used to infect animal cells, expressed a 135-amino-acid beta hCG subunit anchored in cellular membranes by the 48 carboxy-terminal amino acids of vesicular stomatitis virus glycoprotein. The immunogenicity of this recombinant virus with respect to its ability to generate anti-hCG antibodies was compared with that of a second recombinant vaccinia virus expressing the native secretory form of beta hCG. All animals immunized with the vaccinia virus expressing beta hCG on the cell surface elicited high titers of anti-hCG antibodies. Even after a single immunization with the recombinant vaccinia virus, the anti-hCG antibody titers persisted for a long period of time (more than 6 months). None of the animals immunized with vaccinia virus expressing the native secretory form of beta hCG showed any hCG-specific antibody response.

Quantitative export of FGF-2 occurs through an alternative, energy-dependent, non-ER/Golgi pathway

Although basic fibroblast growth factor (bFGF/FGF-2) is found outside cells, it lacks a conventional signal peptide sequence; the mechanism underlying its export from cells is therefore unknown. Using a transient COS-1 cell expression system, we have identified a novel membrane-associated transport pathway that mediates export of FGF-2. This export pathway is specific for the 18-kD isoform of FGF-2, is resistant to the anti-Golgi effects of Brefeldin A, and is energy-dependent. In FGF-2-transfected COS-1 cells, this ER/Golgi-independent pathway appears to be constitutively active and functions to quantitatively export metabolically-labeled 18-kD FGF-2. Co-transfection and co-immunoprecipitation experiments, using a vector encoding the cytoplasmic protein neomycin phosphotransferase, further demonstrated the selectivity of this export pathway for FGF-2. When neomycin phosphotransferase was appended to the COOH-terminus of 18-kD FGF-2, the chimera was exported. However, the transmembrane anchor sequence of the integral membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) blocked export. The chimeric protein localized to the plasma membrane with its FGF-2 domain extracellular and remained cell-associated following alkaline carbonate extraction. Taken together, the data suggest that FGF-2 is "exported" from cells via a unique cellular pathway, which is clearly distinct from classical signal peptide-mediated secretion. This model system provides a basis for the development and testing of therapeutic agents which may block FGF-2 export. Such an intervention may be of considerable use for the treatment of angiogenesis-dependent diseases involving FGF-2.

A mutation in the ectodomain of herpes simplex virus 1 glycoprotein B causes defective processing and retention in the endoplasmic reticulum

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB) is one of several envelope glycoproteins required for virion infectivity and is the only one known to oligomerize into homodimers. To study the conformational constraints for translocation of HSV-1 gB to the surface of eukaryotic cells, we analyzed the transport through the exocytic pathway of the wild-type glycoprotein and of mutant forms with insertions in the ectodomain and intracellular carboxy terminus. Transient expression of the glycoproteins in COS-1 cells showed that an insertion at position 479 in the amino-terminal ectodomain of gB, shown previously by reactions with monoclonal antibodies to have altered the conformation of the molecule, also had a drastic effect on transport, precluding exit of the mutant from the endoplasmic reticulum (ER) and transport to the Golgi and the plasma membrane. The fact that the mutant, gB-(Lk479), formed dimers suggests that local changes in assembled regions caused the transport defect. Mutants containing insertions at residues 600 of the ectodomain and 810 in the intracellular domain were slightly retarded in their rate of transport from the ER to the Golgi. The glucose-regulated proteins GRP78 and GRP94, which are resident proteins of the ER, associated with partially glycosylated, faster-migrating forms of gB but not with the fully processed, more slowly migrating product. GRP78 and GRP94 formed complexes with the mutant gB-(Lk479), which was degraded in the ER. Our results indicate that GRP78, and perhaps also GRP94, acts as a chaperone in the assembly of native gB oligomers and also binds to aberrant forms of the molecule, arresting their transport from the ER and possibly serving as markers for protein degradation in this compartment of the exocytic pathway.

Dissociation and reassociation of oligomeric viral glycoprotein subunits in the endoplasmic reticulum

The vesicular stomatitis virus (VSV) glycoprotein (G) forms noncovalently linked trimers in the endoplasmic reticulum (ER) prior to transport to the cell surface. Here we examined the formation of heterotrimers between wild-type and mutant subunits that were retained in the ER by C-terminal retention signals. When G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate and were transported from the ER at the wild-type rate, heterotrimers were readily detected. In contrast, when G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate, but were retained in the ER, heterotrimers were not detected unless transport of the wild-type molecules from the ER was blocked. After removal of transport block, the heterotrimers then dissociated and reassorted to homotrimers of the mutant protein that were retained in the ER and wild-type trimers that were transported to the cell surface. These and other results presented here indicate that there is an equilibrium between G protein trimers and monomers in vivo, at least in the ER. This equilibrium may function to allow escape of wild-type subunits from aberrant retained subunits.