Baculoviruses are a virus family which probably originated 400 to 450 million years ago and are ubiquitous in the modern environment (Heimpel et al. 1973). Apart from ancient Chinese literature, the earliest evidence of baculoviruses in Western literature can be traced to the sixteenth century by Marco Vida of Cremona describing gory liquefaction of silk worms (reviewed in Benz, 1986). Starting from the 1940’s baculoviruses were used and studied widely as biopestisides in crop fields (Miller, 1997). In the 1930’s a specific baculovirus from Finland was successfully introduced to Canada to abolish spruce sawfly, Gilpinia hercyniae (Arif, 2005; Balch & Bird, 1944). Since the late 80’s and 90’s they have been utilized as production of complex eukaryotic proteins in insect cell cultures (Kost & Condreay, 1999) and later on for viral display and gene therapy(Oker-Blom et al. 2003).
Retroviral particles typically comprise two identical copies of single-stranded, positive sense RNA as a genome, ranging between 7 – 10 kb, which is contained within a protein shell (capsid). The capsid is surrounded by a lipid membrane from which extrudes envelope glycoproteins resulting in a total particle size between 90 and 140 mm. It is the envelope proteins that confer retroviral specificity, as they are responsible for attachment to the target cell allowing for virus internalisation. The envelope-dependent tropism of retroviruses is commonly categorised into three groups; ecotropic (infects murine cells only), xenotropic (infects all cell types except those of murine origin) and amphotropic (infects cells of both murine and non-murine lineage).
Non-viral gene transfer is arguably the safest way in which to introduce exogenous nucleic acid into living cells. However, in general this is also a very inefficient means of introducing DNA or RNA into cells. In order for a gene to be expressed it must find its way into the cell’s nucleus, this means it has to pass two lipid barriers; the extracellular membrane and the nuclear membrane. Given that nucleic acid and the lipids that comprise a cell’s membrane are both negatively charged, electrostatic forces would result in the repulsion of any nucleic acid that approached a cell.
Molecular Biology of the Lentivirus
Lentiviruses are a sub-class of retrovirus that are able to infect both dividing and non-dividing cells. Their genome is much more complex than simple retroviruses, containing an additional six genes; tat, rev, vpr, vpu, nef and vif. Like all retroviruses, lentiviruses are enveloped, however they additionally comprise a pre-integration complex surrounded by the capsid shell.
Molecular Biology of the HSV
Herpes viruses are large, enveloped DNA viruses comprising a genome of double-stranded DNA 150 kb in length located within the viral capsid and encoding some 100-200 genes (see diagram one). Typically, glycoprotein C mediates viral entry by interacting with heparin sulphate, thus initiating a series of interactions ultimately resulting in entry into the cell. Once in the cytoplasm, the viral capsid is transported to nuclear membrane pores where it injects its genome through its capsid portal.
Molecular Biology of the Alphavirus
Alphaviruses consist of an enveloped nucleocapsid containing a single-stranded RNA genome of the positive sense, some 12 kb in length. The viral non-structural proteins, nsP1, nsP2, nsP3 and nsP4 are encoded on the left two-thirds of the genome and are the first viral products to be translated following viral entry. They are initially translated as a polyprotein that is cleaved into four proteins, which form the viral replicase.
Molecular Biology of the Adenovirus
The wild type Adenovirus is a non-enveloped virus comprising a double stranded linear 36 kb genome packaged into an icosahedral protein capsid, of approximately 90-100 nm. Each viral particle has a fiber protein that protrudes from the vertex of the icosahedron, which is attached to a knob region that mediates cell entry. The genome is flanked by two inverted terminal repeat regions of DNA (ITR) and each 5’ end is attached to a 55 kDa terminal protein (TP).
Molecular Biology of the Adeno-Associated Virus
Adeno-associated viruses are non-enveloped and non-pathogenic parvoviruses comprising a double-stranded 5.5 kb genome packaged into a capsid assembly of proteins. At less than 50 nm, this virus is one of the smallest used in gene therapy applications. Consequently, its genome is comprised of only two genes, rep and cap encoding for non-structural and structural genes respectively, which are flanked by two 145 bp ITRs (see figure one).