The image shows a single-particle electron microscope tomography reconstruction, which reveals that a fully assembled drug-loaded nanodisk (red) can be packaged into the vault lumen (green) as a viable method for vault-mediated drug delivery. The electron micrograph in the background shows negatively stained vaults containing nanodisks.
The development of new methods that use engineered nanomaterials to transport drugs and release them directly into cells holds great potential in this area. And while several such drug-delivery systems — including some that use dendrimers, liposomes or polyethylene glycol — have won approval for clinical use, they have been hampered by size limitations and ineffectiveness in accurately targeting tissues. UCLA demonstrated the ability to package drug-loaded “nanodisks” into vault nanoparticles, naturally occurring nanoscale capsules that have been engineered for therapeutic drug delivery.
The vault nanoparticle is one of the largest known ribonucleoprotein complexes in the sub-100 nm range. Highly conserved and almost ubiquitously expressed in eukaryotes, vaults form a large nanocapsule with a barrel-shaped morphology surrounding a large hollow interior. These properties make vaults an ideal candidate for development into a drug delivery vehicle. In this study, the first example of using vaults towards this goal is reported. Recombinant vaults are engineered to encapsulate the highly insoluble and toxic hydrophobic compound all-trans retinoic acid (ATRA) using a vault-binding lipoprotein complex that forms a lipid bilayer nanodisk. These recombinant vaults offer protection to the encapsulated ATRA from external elements. Furthermore, a cryo-electron tomography (cryo-ET) reconstruction shows the vault-binding lipoprotein complex sequestered within the vault lumen. Finally, these ATRA-loaded vaults show enhanced cytotoxicity against the hepatocellular carcinoma cell line HepG2. The ability to package therapeutic compounds into the vault is an important achievement toward their development into a viable and versatile platform for drug delivery.
Vault nanoparticles are found in the cytoplasm of all mammalian cells and are one of the largest known ribonucleoprotein complexes in the sub-100-nanometer range. A vault is essentially barrel-shaped nanocapsule with a large, hollow interior — properties that make them ripe for engineering into a drug-delivery vehicles. The ability to encapsulate small-molecule therapeutic compounds into vaults is critical to their development for drug delivery.
Recombinant vaults are nonimmunogenic and have undergone significant engineering, including cell-surface receptor targeting and the encapsulation of a wide variety of proteins.
“A vault is a naturally occurring protein particle and so it causes no harm to the body,” said Rome, CNSI associate director and a professor of biological chemistry. “These vaults release therapeutics slowly, like a strainer, through tiny, tiny holes, which provides great flexibility for drug delivery.”
The internal cavity of the recombinant vault nanoparticle is large enough to hold hundreds of drugs, and because vaults are the size of small microbes, a vault particle containing drugs can easily be taken up into targeted cells.
With the goal of creating a vault capable of encapsulating therapeutic compounds for drug delivery, UCLA doctoral student Daniel Buhler designed a strategy to package another nanoparticle, known as a nanodisk (ND), into the vault’s inner cavity, or lumen.
“By packaging drug-loaded NDs into the vault lumen, the ND and its contents would be shielded from the external medium,” Buehler said. “Moreover, given the large vault interior, it is conceivable that multiple NDs could be packaged, which would considerably increase the localized drug concentration.”
According to researcher Zhou, a professor of microbiology, immunology and molecular genetics and director of the CNSI’s Electron Imaging Center for NanoMachines, electron microscopy and X-ray crystallography studies have revealed that both endogenous and recombinant vaults have a thin protein shell enclosing a large internal volume of about 100,000 cubic nanometers, which could potentially hold hundreds to thousands of small-molecular-weight compounds.
“These features make recombinant vaults an attractive target for engineering as a platform for drug delivery,” Zhou said. “Our study represents the first example of using vaults toward this goal.”
“Vaults can have a broad nanosystems application as malleable nanocapsules,” Rome added.
The recombinant vaults are engineered to encapsulate the highly insoluble and toxic hydrophobic compound all-trans retinoic acid (ATRA) using a vault-binding lipoprotein complex that forms a lipid bilayer nanodisk.
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