Adeno-associated Virus - AAV Infection Cycle

AAV Infection Cycle

There are several steps in the AAV infection cycle, from infecting a cell to producing new infectious particles:

1. attachment to the cell membrane
2. endocytosis
3. endosomal trafficking
4. escape from the late endosome or lysosome
5. translocation to the nucleus
6. formation of double-stranded DNA replicative form of the AAV genome
7. rep genes expression
8. genome replication
9. cap genes expression, synthesis of progeny ssDNA particles
10. assembly of complete virions, and
11. release from the infected cell.

Some of these steps may look different in various types of cells, which, in part, contributes to the defined and quite limited native tropism of AAV. Replication of the virus can also vary in one cell type, depending on the cell's current cell cycle phase.

The characteristic feature of the adeno-associated virus is a deficiency in replication and thus its inability to multiply in unaffected cells. The first factor that was described as providing successful generation of new AAV particles, was the adenovirus, from which the AAV name originated. It was then shown that AAV replication can be facilitated by selected proteins derived from the adenovirus genome, by other viruses such as HSV, or by genotoxic agents, such as UV irradiation or hydroxyurea.

The minimal set of the adenoviral genes required for efficient generation of progeny AAV particles, was discovered by Matsushita, Ellinger et al.. This discovery allowed for new production methods of recombinant AAV, which do not require adenoviral co-infection of the AAV-producing cells. In the absence of helper virus or genotoxic factors, AAV DNA can either integrate into the host genome or persist in episomal form. In the former case integration is mediated by Rep78 and Rep68 proteins and requires the presence of ITRs flanking the region being integrated. In mice, the AAV genome has been observed persisting for long periods of time in quiescent tissues, such as skeletal muscles, in episomal form (a circular head-to-tail conformation).