Structure reveals how AAV2 bundles actin filaments, offering new clues for gene therapy capsid design
Direct interaction between AAV2 en route to the nucleus and actin
A study reports that adeno-associated virus serotype 2 (AAV2), used as a gene therapy vector, binds directly to actin filaments and bundles multiple filaments into a periodic hexagonal lattice. The researchers analyzed the binding structure using cryo-electron microscopy (cryo-EM) and also created a capsid variant that does not bundle actin. However, the idea that this interaction traps capsids and mediates their accumulation near the nucleus is a possibility proposed by the researchers; it does not mean that improved nuclear transport or therapeutic efficacy has been demonstrated.
An AAV capsid is the viral protein shell that encloses its genetic material. The researchers explained that AAV is used for single-dose treatment of monogenic diseases and for gene expression lasting several years, but that low transduction efficiency means high doses are needed to achieve therapeutic effects. Transduction is the process of delivering genetic material into cells using a vector. The cost of high doses and rare but life-threatening immune responses provided the context for the study. Important aspects of how capsids travel to the nucleus after endocytosis, the process by which cells take up external material, remain to be clarified.
Joint analysis of actin-bundling structure and function
Actin filaments are fibrous structures that form part of the cytoskeleton. This study reports that AAV2 not only interacts directly with these structures but also bundles multiple filaments into a regularly repeating hexagonal lattice. This offers a way to examine capsid–cytoskeleton contact from both structural and functional perspectives.
Using cryo-EM, the team determined the binding structure of AAV2 and actin filaments and identified the interaction sites. They then created a capsid variant that had lost its actin-bundling function. The findings for the variant reported in the abstract are limited to loss of this bundling function. The finding that it does not bundle actin cannot, by itself, be taken to mean that all binding to actin has been eliminated or that its ability to enter the nucleus has improved.
Based on these properties, the researchers proposed a new class of actin-binding proteins called “actin bundling viruses” (ABVs), with AAV2 as its first member. This is the researchers’ proposed classification; the results do not show that all AAV types have the same function.
A hypothesis for perinuclear accumulation, and questions still to test
The researchers interpreted the AAV2–actin interaction as possibly trapping capsids and mediating their accumulation near the nucleus. This hypothesis provides a reason to consider intracellular interactions in capsid design. However, the provided abstract contains no quantitative comparisons between the variant and the original capsid for transport speed to the nucleus, nuclear entry rate, or gene delivery efficiency. It also does not establish accelerated transport along microtubules or resolution of perinuclear accumulation.
This is a study of structure and interactions presented as a bioRxiv preprint. The material reviewed was limited to the abstract, so details of the experimental system, sample sizes, comparison conditions, and observation period could not be confirmed. The results should therefore be viewed as a new research direction for AAV2 capsid design. Further validation is needed to determine whether they could lead to reduced doses, reduced toxicity, or lower manufacturing costs.
The core findings are available in the PubMed-indexed abstract of “AAV2 Crosslinks Actin Filaments: Implications for AAV Gene Therapy Vector Design”.
Adeno-associated virus (AAV) capsids are important gene therapy vectors, allowing for the one-time treatment of monogenetic disorders, with durable gene expression lasting for years. However, despite the clinical success of AAV usage, low transduction efficiencies require high dosage to achieve therapeutic efficacy, resulting in prohibitive costs and rare, but life-threatening immune responses. One key knowledge gap is how the capsids traffic to the nucleus following endocytosis. Here, we identify a direct interaction between AAV2 and actin filaments. Our results show that AAV2 bundles multiple actin filaments in a highly periodic hexagonal lattice. Therefore, we propose that AAV2 is the founding member of a new class of actin binding proteins, the actin bundling viruses (ABVs). Using cryogenic electron microscopy, we determined the structure of AAV2-actin filaments, identified the interaction interfaces, and engineered capsid variants that no longer bundle actin. Together, our results suggest that the AAV2-actin interaction may be responsible for trapping capsids and mediating peri-nuclear accumulation. Overall, this interaction expands the current understanding of AAV2 biology and offers new directions for capsid engineering.
The practical value of this study lies in its structural analysis of the direct interaction between the AAV2 capsid and actin, and in the creation of a variant with altered actin-bundling function. Such a variant could provide a starting point for follow-up studies comparing the role of bundling in perinuclear accumulation and gene delivery. Loss of bundling, however, does not by itself mean improved delivery efficiency. To use this as a design strategy, researchers would need to compare the intracellular distribution, nuclear entry, and gene expression of the original capsid and the variant under the same conditions, and assess safety separately. The current evidence introduces a new design variable; it does not establish a low-dose, high-efficacy vector or validated toxicity metrics.