Transient expression of a mitochondrial precursor protein. A new approach to study mitochondrial protein import in cells of higher eukaryotes

Semliki forest virus-based expression for versatile use in receptor research

Semliki Forest virus (SFV) vectors have been generated for highly efficient studies on gene expression in a variety of mammalian host cells, including immortalized cell lines as well as primary cells in culture. Moreover, SFV expression has been scaled up for mammalian suspension cultures in spinner flasks and bioreactors for production of large quantities of recombinant proteins for drug screening and purification. The strong preference of expression in neuronal cells in primary cell cultures, in organotypic hippocampal slices and in vivo has made SFV vectors attractive for neurobiological studies. Additionally, the engineering of novel, less cytotoxic and temperature-sensitive SFV mutant vectors has further increased their application range.

Tom20-mediated mitochondrial protein import in muscle cells during differentiation

Mitochondrial biogenesis is accompanied by an increased expression of components of the protein import machinery, as well as increased import of proteins destined for the matrix. We evaluated the role of the outer membrane receptor Tom20 by varying its expression and measuring changes in the import of malate dehydrogenase (MDH) in differentiating C2C12 muscle cells. Cells transfected with Tom20 had levels that were twofold higher than in control cells. Labeling of cells followed by immunoprecipitation of MDH revealed equivalent increases in MDH import. This parallelism between import rate and Tom20 levels was also evident as a result of thyroid hormone treatment. Using antisense oligodeoxynucleotides, we inhibited Tom20 expression by 40%, resulting in 40-60% reductions in MDH import. In vitro assays also revealed that import into the matrix was more sensitive to Tom20 inhibition than import into the outer membrane. These data indicate a close relationship between induced changes in Tom20 and the import of a matrix protein, suggesting that Tom20 is involved in determining the kinetics of import. However, this relationship was dissociated during normal differentiation, since the expression of Tom20 remained relatively constant, whereas imported MDH increased 12-fold. Thus Tom20 is important in determining import during organelle biogenesis, but other mechanisms (e.g., intramitochondrial protein degradation or nuclear transcription) likely also play a role in establishing the final mitochondrial phenotype during normal muscle differentiation.

Recombinant adeno-associated virus vector-based gene transfer for defects in oxidative metabolism

Defects in oxidative metabolism may be caused by mutations either in nuclear genes or in mitochondrial DNA (mtDNA). We tested the hypothesis that recombinant adeno-associated virus (rAAV) could be used to complement mtDNA mutations. AAV vector constructs were designed to express the reporter gene encoding green fluorescent protein (GFP), fused to a targeting presequence that directed GFP to be translocated into mitochondria. These vectors mediated expression of mitochondrial-localized GFP, as indicated by fluorescence microscopy and electron microscopy, in respiring human embryonic kidney 293 cells and nonrespiring mtDNA-deficient (rho 0) cells. However, when sequences encoding hydrophobic segments of proteins normally encoded by mtDNA were inserted between the presequence and GFP, mitochondrial import failed to occur. In similar experiments, a fusion was created between pyruvate dehydrogenase (PDH) E1 alpha subunit, a nuclear-encoded mitochondrial gene with its own targeting presequence, and GFP. With this construct, expression of GFP was observed in mitochondria in vitro and in vivo. We conclude that the hydrophobicity of mtDNA-encoded proteins limits their ability to be transported from the cytoplasm. However, rAAV-based gene therapy may hold promise for gene therapy of PDH deficiency, the most common biochemically proven cause of congenital lactic acidosis.

Large scale transient 5-HT3 receptor production with the Semliki Forest Virus Expression System

The expression of recombinant proteins with the Semliki Forest Virus (SFV) system has been scaled up to bioreactor scale. As a model protein for this study the human 5-HT(3) receptor was chosen. The gene for the receptor was subcloned into the SFV expression plasmid pSFV1. Virus production by in vivo packaging and production of the recombinant protein was scaled up, the latter to a reactor volume of 11.5 l. A Vibromix(TM) agitation system was chosen to overcome aggregation problems of BHK cells in suspension. In the process, cells were first grown to a density of 10(6) cells/ml, the medium was then exchanged with fresh medium and the culture was infected with the recombinant virus at an estimated multiplicity of infection of 30. 24 h post infection we measured an expression level of 3 million functional 5-HT(3) receptors per cell. For harvesting, the cells were pelleted by centrifugation. The receptor protein was purified in a single step (Hovius et al., 1998) by exploiting the hexa-His tag at minimal protein loss (51% yield). Experiments to optimise expression resulted in yields up to 8 million receptors per cell, when the pH of a suspension culture was controlled at pH 7.3. Rapid virus generation and protein production, high protein yields as well as successful large scale application have made the SFV expression system attractive to produce large quantities of recombinant protein in a very short time. After optimisation of the expression conditions (in particular by setting the pH at 7.3), yields were increased twofold.

Targeting of a human iron-sulfur cluster assembly enzyme, nifs, to different subcellular compartments is regulated through alternative AUG utilization

Iron-sulfur clusters are prosthetic groups that are required for the function of numerous enzymes in the cell, including enzymes important in respiration, photosynthesis, and nitrogen fixation. Here we report cloning of the human homolog of NifS, a cysteine desulfurase that is proposed to supply the inorganic sulfur in iron-sulfur clusters. In human cells, different forms of NifS that localize either to mitochondria or to the cytosol and nucleus are synthesized from a single transcript through initiation at alternative inframe AUGs, and initiation site selection varies according to the pH of the medium or cytosol. Thus, a novel form of translational regulation permits rapid redistribution of NifS proteins into different compartments of the cell in response to changes in metabolic status.

Alphaviruses as expression vectors

Alphavirus vectors have been used for efficient high-level expression of a variety of topologically different proteins, allowing studies of protein transport, localization and functional activity in a broad range of host cells. Complex transmembrane proteins have been produced in large quantities through the establishment of scale-up technology. Alphavirus vectors have also shown promising potential in vaccine production and preliminary gene therapy applications.

Generation of heat shock protein-based vaccines by intracellular loading of gp96 with antigenic peptides

Several studies have shown that immunization with heat shock proteins (HSPs) purified from tumors of virus-infected cells induces specific cytotoxic T-cell (CTL) activity. This immune response is directed against peptides bound to the HSPs rather than against the HSPs themselves. The peptides are derived from tumor- or virus-specific proteins which are degraded in the course of normal protein turnover and processing for presentation by MHC class I molecules. The HSPs appear to function as carriers for the antigenic peptides. Upon immunization they ensure their uptake by specialized macrophages and their introduction into the MHC class I presentation route which is otherwise accessible only for intracellular proteins. Using influenza virus nucleoprotein (NP) as a model antigen, we have tested whether an HSP-based vaccine can be preduced by overexpressing an antigen in cultured cells prior to purification of the HSP's. The transfection system based on the Semliki Forest virus (SFV) replicon was employed to achieve high expression of NP. Since SFV-mediated transfection of murine cells was inefficient we used the hamster-derived cell line BHK21, which can be transfected with 100% efficiency, as a source for NP peptide-loaded gp96. The protein was purified from transfected cells and used for first vaccination studies. The hamster gp96 preparation was well tolerated in mice, an antibody response against the foreign protein was not observed. Preliminary results suggest that a cellular immune response against NP was indeed induced. SFV transfection is applicable for any known antigen and is therefore considered to be an elegant means for the production of HSP-based vaccines capable of inducing a cellular immune response.

Protein import into mitochondria

Mitochondria import many hundreds of different proteins that are encoded by nuclear genes. These proteins are targeted to the mitochondria, translocated through the mitochondrial membranes, and sorted to the different mitochondrial subcompartments. Separate translocases in the mitochondrial outer membrane (TOM complex) and in the inner membrane (TIM complex) facilitate recognition of preproteins and transport across the two membranes. Factors in the cytosol assist in targeting of preproteins. Protein components in the matrix partake in energetically driving translocation in a reaction that depends on the membrane potential and matrix-ATP. Molecular chaperones in the matrix exert multiple functions in translocation, sorting, folding, and assembly of newly imported proteins.