Viral Vectors

Over the past few years, several viral vectors with low toxicity, high infection rate, and persistent expression have extended the methodology of delivery of genes to mammalian cells. These viruses include DNA viruses, such as adenoviruses and adeno-associated viruses, herpes simplex viruses, and RNA retroviruses. Recently, as a result of advances in genetic manipulation, adenoviruses and adeno-associated viruses have been more widely applied to gene transfer. The advantages of adenoviruses are 1) the ability to carry large sequences of foreign DNA, 2) the ability to infect a broad range of cell types, and 3) an almost 100% expression of the foreign gene in cells.

Human adenovirus is a large DNA virus (36 kb of DNA) composed of early genes (from E1 to E4) and later genes (L1 to L5). Wild-type adenovirus cannot be applied to gene transfer, because it causes a lytic infection. Thus, recombinant adenoviruses with defects of some essential viral genes are used for gene delivery. These recently developed adenoviral expression systems are safe; such systems have the capacity for large DNA inserts and allow for relatively simple adenoviral production (Harding et al. 1997; He et al. 1998).

The process of gene transfer into cells (cell lines and primary cells) via recombinant adenoviruses is simple, but the optimal viral titer, the time of exposure to virus, and the multiplicity of infection should be optimized for each cell type. Cell lines and a variety of primary neuronal cells have been infected by adenoviruses (Barkats et al. 1996; Chen and Lambert 2000; Hughes et al. 2002; Koshimizu et al. 2002; Slack et al. 1996). In addition, recombinant adenoviruses containing the desired genes can be delivered to neurons in vivo via intracerebral injection into particular brain areas (Bemelmans et al. 1999; Benraiss et al. 2001; Berry et al. 2001; Neve 1993).

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