Low Voltage Electron Microscopy for the Characterization of Adeno-Associated Virus (AAV)

Low Voltage Electron Microscopy provides a cost-effective alternative for Adeno-Associated Virus (AAV) analysis compared to cryo-TEM.

rAAV is essential for gene replacement therapy, delivering functional genes to targeted tissues. Low Voltage Electron Microscopy (LVEM) offers significant potential for analyzing the structure and quality of AAV capsids effectively. Gene therapy aims to correct genetic defects by delivering functional copies of genes to target tissues, often using viral vectors like AAV. These vectors consist of a 27 nm diameter capsid that encapsulates the therapeutic gene. Electron microscopy, including Cryogenic Transmission Electron Microscopy (cryo-TEM), is commonly used to analyze these viral particles. However, these methods are typically challenging and requires large and expensive specialized equipment and conditions.

Figure A. Cryo-TEM. Distinguishes empty and full vectors, resolves some structure. Figure B. Unstained LVEM. Distinguishes empty and full vectors, cannot see structure. Figure C. Negatively Stained LVEM & HV-TEM. Less effective at differentiating empty and full vectors, resolves structure.

Low voltage electron microscopy (LVEM) offers a compact and affordable alternative for AAV capsid characterization. Operating at a lower voltage (25 kV compared to 100 kV or more), the LVEM 25 from Delong Instruments can provide high-contrast images with a resolution of up to 1 nm, making it particularly useful for analyzing AAV samples in gene therapy.

Comparison of the cryo-TEM and LVEM techniques demonstrates that both methods can distinguish between empty capsids (ECs), full capsids (FCs), as well as visualize partially filled capsids (PCs) and high-molecular-weight (HMW) species. This discrimination is necessary to understand and improve the success of the capsid purification process which aims to optimize the ratio of full capsids.

The LVEM 25’s ability to provide detailed visualization at lower voltages is particularly advantageous for examining small, delicate structures. Detailed structural information, such as identifying dimers and trimers, can be done using classic negative stain technique. Unique to LVEM, the unstained imaging technique can be used to provide more reliable discrimination of ECs, PCs, and FCs. Since staining permeability within an AAV population is variable, stained imaging is unreliable for empty/full analysis. The unstained imaging technique avoids artifacts introduced by staining and therefore holds a strong advantage for quantifying this ratio. Sample preparation for LVEM is far simpler than that for cryo-TEM, and merely involves depositing the sample onto a TEM grid, washing away excess buffer and allowing the sample to dry.

The LVEM 25’s key advantages are its ability to perform unstained imaging, in a compact and lower cost instrument as compared to cryo-TEM or higher energy TEMs. Unstained imaging provides a more accurate representation of viral particles, allowing for a more reliable identification of full and empty capsids.

The LVEM 25 is an excellent instrument for characterizing AAV samples, offering a compact and cost-effective alternative to cryo-TEM for many applications, except for those requiring high-resolution structure determination. Futhermore, due to LVEM’s accessibility and affordability, it is a superior characterization method to outsourced cryo-TEM. Its ability to visualize and quantify broken capsids, ECs, PCs, FCs, and HMW species makes LVEM an essential tool in gene therapy research.

Ausman, K. D., Whitaker, N., Balasubramanian, M., Kokona, B., Vogt, A., & Kar, S. R. (2025). Low voltage electron microscopy: An emerging tool for AAV characterization. Pharmaceutical Biotechnology. https://doi.org/10.1016/j.xphs.2025.01.013