Mammalian Expression Vectors

Expression systems utilizing mammalian cells can perform appropriate protein folding, complex N-linked glycosylation, authentic O-linked glycosylation, and extensive post-translational modifications, which are important for complete biological activity of proteins. The gene expression in mammalian cells requires a suitable cell line and appropriate vectors. These vectors introduce specific genes into mammalian cells, transcribe those genes, and subsequently produce modified proteins. Mammalian expression vector systems promote constitutive and inducible expression of post-translational modified and appropriately folded proteins. In addition, these vectors can characterize the impact of specific mutations on cell metabolism, and stably alter cellular phenotype as a function of transgene expression.

Mammalian expression vectors are generally divided into two categories, namely viral vectors and plasmid vectors. Viral vectors are essentially inactivated viruses into which genes are cloned, such as adenoviruses, retroviruses, and lentiviruses. Plasmid vectors are more complex, and their functional components include prokaryotic, eukaryotic, and viral sequences. Prokaryotic elements facilitate bacterial propagation and vector maintenance, and eukaryotic and viral elements comprise transcription elements and sequence coding for selectable markers. Transcriptional regulation involves the coordinated interaction of multiple genetic elements. The regulation of gene expression in a plasmid or viral vector is controlled by promoter and enhancer sequences as well as signal sequences required for efficient processing of transcripts. The most common promoters and enhancers are of viral origin. Locus control regions (LCRs) can be used in specific cell types to regulate gene expression. In addition to transcriptional control elements and translational control elements, mammalian expression vectors contain selectable markers for vector propagation in bacteria, as well as markers for selection of transfected cells, preparation of stable cell lines, and gene amplification.

When designing a DNA transfer and expression study, the primary factors to consider in selecting expression vectors are the type of promoter/enhancer sequence, transient or stable expression type, and the desired protein expression amount.