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Researchers from Rice University have found a way to deliver peptide drugs to cells and tissues using a virus-like nanoscale device.

By chipping away at one of the three proteins that make up the protective shell called the capsid of natural adeno-associated viruses (AAV), the researchers made the capsids with unique abilities that could lead to a better drug delivery system.

AAVs are about 25 nanometers and contain a single strand of DNA inside tough capsids that consist of a mosaic of proteins known as VP1, VP2 and VP3. AAVs are often used to deliver gene-therapy payloads.

However, it is unknown how AAV capsids physically reconfigure themselves when triggered by external stimuli.

“This virus has intrinsic peptide [small protein] domains hidden inside the capsid,” Rice bioengineer Junghae Suh said in a statement. “When the virus infects a cell, it senses the low pH and other endosomal factors, and these peptide domains pop out onto the surface of the virus capsid.

“This conformational change, which we termed an 'activatable peptide display,' is important for the virus because the externalized domains break down the endosomal membrane and allow the virus to escape into the cytoplasm,” she added. “In addition, nuclear localization sequences in those domains allow the virus to transit into the nucleus. We believed we could replace that functionality with something else.”

The researchers believe that the mutant AAVs can become biocomputing nanoparticles that can detect and process environmental inputs and produce controllable outputs, with the modified capsids being the initial step.

Despite being unable to make a capsid on its own both VP1 and VP2 can be triggered to expose their functional peptides.

Shorter VP3s can form capsids by themselves, but do not display peptides. In natural AAVs, VP3 proteins outnumber each of their compadres 10-to-1, which limits the number of peptides that can be exposed.

The researchers truncated VP2 and synthesized mosaic capsids with VP3 to alter the number of exposed peptides. They then inserted a common hexahistidine tag that made it easy to monitor the surface display of the peptide region.

“We wanted to boost the protein's activable property beyond what occurs in the native virus capsid,” Rice graduate student Nicole Thadani said in a statement. “Rather than displaying just five copies of the peptide per capsid, now we may be able to display 20 or 30 and get more of the bioactivity that we want.”

The researchers then made a truncated VP2 able to form a capsid on its own.

“We chopped down that VP2 component enough to form what we call a homomeric capsid, where the entire capsid is made up of just that mutant subunit,” Suh said. “That gave us viruses that appear to have peptide 'brushes' that are always on the surface.

“A viral structure like that has never been seen in nature,” she added. “We got a particle with this peptide brush, with loose ends everywhere. Now we want to know if we can use these loose ends to attach other things or carry out other functions.”

Homomeric AAVs can display as many as 60 peptides, while mosaic AAVs could be programmed to respond to stimuli specific to particular cells or tissues and display a smaller desired number of peptides.

“Viruses have evolved to invade cells very effectively,” Suh said. “We want to use our virus as a nanoparticle platform to deliver protein- or peptide-based therapeutics more efficiently into cells.

“We want to harness what nature has already created, tweak it a little bit and use it for our purposes,” she added.

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