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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Adeno-associated virus | 3/5 | https://en.wikipedia.org/wiki/Adeno-associated_virus | reference | science, encyclopedia | 2026-05-05T14:17:29.244416+00:00 | kb-cron |
The cap gene produces an additional, non-structural protein called the Assembly-Activating Protein (AAP). This protein is produced from ORF2 and is essential for the capsid-assembly process. The exact function of this protein in the assembly process and its structure have not been solved to date. All three VPs are translated from one mRNA. After this mRNA is synthesized, it can be spliced in two different manners: either a longer or shorter intron can be excised resulting in the formation of two pools of mRNAs: a 2.3 kb- and a 2.6 kb-long mRNA pool. Usually, especially in the presence of adenovirus, the longer intron is preferred, so the 2.3-kb-long mRNA represents the so-called "major splice". In this form the first AUG codon, from which the synthesis of VP1 protein starts, is cut out, resulting in a reduced overall level of VP1 protein synthesis. The first AUG codon that remains in the major splice is the initiation codon for VP3 protein. However, upstream of that codon in the same open reading frame lies an ACG sequence (encoding threonine) which is surrounded by an optimal Kozak context. This contributes to a low level of synthesis of VP2 protein, which is actually VP3 protein with additional N terminal residues, as is VP1. Since the bigger intron is preferred to be spliced out, and since in the major splice the ACG codon is a much weaker translation initiation signal, the ratio at which the AAV structural proteins are synthesized in vivo is about 1:1:20, which is the same as in the mature virus particle. The unique fragment at the N terminus of VP1 protein was shown to possess the phospholipase A2 (PLA2) activity, which is probably required for the releasing of AAV particles from late endosomes. Muralidhar et al. reported that VP2 and VP3 are crucial for correct virion assembly. More recently, however, Warrington et al. showed VP2 to be unnecessary for the complete virus particle formation and an efficient infectivity, and also presented that VP2 can tolerate large insertions in its N terminus, while VP1 can not, probably because of the PLA2 domain presence.
==== Post-translational modifications ==== Recent discoveries made through use of high-throughput 'omics approaches include the fact that AAV capsids are post-translationally modified (PTM) during production such as acetylation, methylation, phosphorylation, deamidation, O-GlycNAcylation and SUMOylation throughout capsid proteins VP1, VP2 and VP3. These PTMs differ depending on the manufacturing production platform. Another such discovery is the fact that AAV genomes are epigenetically methylated during production. Besides price, these findings might affect expression kinetics, rAAV receptor binding, trafficking, vector immunogenicity, and expression durability.
== Classification, serotypes, receptors and native tropism ==
Two species of AAV were recognised by the International Committee on Taxonomy of Viruses in 2013: adeno-associated dependoparvovirus A (formerly AAV-1, −2, −3 and −4) and adeno-associated dependoparvovirus B (formerly AAV-5). Until the 1990s, virtually all AAV biology was studied using AAV serotype 2. However, AAV is highly prevalent in humans and other primates and several serotypes have been isolated from various tissue samples. Serotypes 2, 3, 5, and 6 were discovered in human cells, AAV serotypes 1, 4, and 7–11 in nonhuman primate samples. As of 2006 there have been 11 AAV serotypes described, the 11th in 2004. AAV capsid proteins contain 12 hypervariable surface regions, with most variability occurring in the threefold proximal peaks, but the parvovirus genome in general presents highly conserved replication and structural genes across serotypes. All of the known serotypes can infect cells from multiple diverse tissue types.
=== Serotype 2 === Serotype 2 (AAV2) has been the most extensively examined so far. AAV2 presents natural tropism towards skeletal muscles, neurons, vascular smooth muscle cells and hepatocytes. Three cell receptors have been described for AAV2: heparan sulfate proteoglycan (HSPG), aVβ5 integrin and fibroblast growth factor receptor 1 (FGFR-1). The first functions as a primary receptor, while the latter two have a co-receptor activity and enable AAV to enter the cell by receptor-mediated endocytosis. These study results have been disputed by Qiu, Handa, et al. HSPG functions as the primary receptor, though its abundance in the extracellular matrix can scavenge AAV particles and impair the infection efficiency. Studies have shown that serotype 2 of the virus (AAV-2) apparently kills cancer cells without harming healthy ones. "Our results suggest that adeno-associated virus type 2, which infects the majority of the population but has no known ill effects, kills multiple types of cancer cells yet has no effect on healthy cells," said Craig Meyers, a professor of immunology and microbiology at the Penn State College of Medicine in Pennsylvania in 2005. This could lead to a new anti-cancer agent. In March 2023, a series of Nature papers linked infection of adeno-associated virus 2 (AAV2) to a wave of childhood hepatitis.
=== Other serotypes ===
Although AAV2 is the most popular serotype in various AAV-based research, it has been shown that other serotypes can be more effective as gene delivery vectors. AAV9 passes the blood-brain-barrier in humans, AAV6 appears much better in infecting airway epithelial cells, AAV7 presents very high transduction rate of murine skeletal muscle cells (similar to AAV1 and AAV5), AAV8 transduce hepatocytes and AAV1 and 5 were shown to be very efficient in gene delivery to vascular endothelial cells. In the brain, most AAV serotypes show neuronal tropism, while AAV5 also transduces astrocytes. AAV6, a hybrid of AAV1 and AAV2, also shows lower immunogenicity than AAV2. Serotypes can differ with the respect to the receptors they are bound to. For example, AAV4 and AAV5 transduction can be inhibited by soluble sialic acids (of different form for each of these serotypes), and AAV5 was shown to enter cells via the platelet-derived growth factor receptor.