Construction of Adenovirus Vectors for Studies of Protein Function and RNA Interference

Abstract: During an adenovirus infection the accumulation of alternatively spliced mRNAs is subjected to a tight temporal regulation. The IIIa protein is a structural protein expressed exclusively late after infection. To study the significance of the restricted IIIa protein expression we used a Tet-ON regulated adenoviral vector to overexpress the IIIa protein during the early phase of infection. The results show that unregulated IIIa protein expression caused a reduction in late viral protein accumulation and a slight block of viral DNA replication. Further, the results indicate that IIIa splicing might be subjected to a regulation via a feed back loop stimulating its own expression.To improve the efficacy of vectors for regulated transgene expression, we constructed binary adenoviral vectors based on the Tet-ON and Tet-OFF systems. These vectors encode both the transcriptional activator and the tetracycline-regulated expression cassette from the same viral unit, ensuring that each infected cell will express both the activator and the reporter gene. In model experiments this system was shown to result in a tight control of gene expression with no detectable background expression of the transgene and induction levels reaching 500-600 fold.Introduction of dsRNA into a cell will induce a sequence specific degradation of the homologous mRNA via a mechanism named RNA interference (RNAi). The adenovirus VA RNAs are short highly structured RNAs that are expressed in large amounts late during an adenovirus infection. Here we showed that both VA RNAI and VA RNAII functions as virus-encoded suppressors of RNAi, by interfering with the activity of Dicer, the enzyme that cleaves the initial dsRNA to short-interfering RNAs (siRNAs) that mediate RNAi. Further, the VA RNAs themselves are substrates for Dicer and are cleaved into siRNAs in vivo that are incorporated into active RNA-induced silencing complexes.There is a great interest in developing novel therapeutic strategies based on RNAi. We constructed adenoviral vectors that express short hairpin RNAs, which in vivo will be cleaved to siRNAs that induce sequence-specific RNAi. We compared the efficiency of RNAi induced by vectors based on the viral VA RNAI and the human U6 promoters. Our results suggest that under conditions where the recombinant virus does not replicate, the VA RNA promoter is more effective in down regulating target gene expression, whereas the U6 promoter was more effective under replicative conditions.