Extracellular vesicles (EVs) could serve as outstanding drug delivery vehicles. Their low toxicity and low immunogenicity would make them useful in sustained treatment or redosing scenarios. But there’s a problem. The loading of EVs is difficult. The EVs can lose structural integrity, and the therapeutic cargo that is put into the EVs can lose activity.
To address these problems, scientists at Northwestern University studied how proteins associate with the plasma membrane and lipid rafts of EVs. The scientists used membrane spectroscopy and bioinformatics to confirm that EVs are lipid raft-like in membrane order and protein composition, respectively. They used bioinformatic analysis to better understand the physical properties characteristic of proteins naturally found in EVs. And they generated a library of transmembrane and peripheral proteins predicted to load well or poorly into EVs. This work allowed them to generate a library of transmembrane and peripheral proteins predicted to load well or poorly into EVs.
The scientists presented their findings in Nature Communications, in an article titled, “Enhancing extracellular vesicle cargo loading and functional delivery by engineering protein-lipid interactions.”
“[We] demonstrate that by rationally designing proteins to traffic to the plasma membrane and associate with lipid rafts, we can enhance loading of protein cargo into EVs for a set of structurally diverse transmembrane and peripheral membrane proteins,” the article’s authors wrote. “We then demonstrate the capacity of select lipid tags to mediate increased EV loading and functional delivery of an engineered transcription factor to modulate gene expression in target cells.”
EVs are natural, lipid-based nanoparticles that deliver biological cargo and mediate natural intercellular communication. With a little help, they could be harnessed to move biological drugs—molecules like proteins, RNA, and combinations of the two—to specific diseased parts of the body.
The Northwestern University scientists sought to effectively deliver an engineered protein to its target cell and trigger a change in the cell’s gene expression. And they did so by using proteins that they had engineered to “traffic to and associate with” a specific cell membrane structure that the researchers found increased a protein’s likelihood of latching onto the container.
The success brings researchers a step closer to addressing a major bottleneck for biological medicine development, determining how to protect fragile molecules in the body and ensure they reach the correct diseased cells in a patient without impacting healthy cells.
The research combines work from two labs in Northwestern’s Center for Synthetic Biology: those of biomedical engineer Neha Kamat, PhD, and chemical and biological engineer Josh Leonard, PhD. The Kamat lab has largely focused on the design of synthetic containers and uses biophysical principles to control molecules targeting other cells. Leonard’s lab develops tools to build engineered EVs.
“We were interested in applying some of the biophysical insights that have emerged about how to localize proteins to specific membrane structures so that we could hijack this natural system,” said Kamat, the paper’s co-corresponding author and associate professor at the McCormick School of Engineering. “In this study, we discover general ways to load drug cargo into these vesicles very efficiently while preserving their function. This might enable more effective and affordable extracellular vesicle-based biological medicines.”
The keys to this “cargo loading” approach are sites on cell membranes called lipid rafts. These regions are more structured than the rest of the membrane and reliably contain specific proteins and lipids.
The article’s lead authors are Justin Peruzzi and Taylor Gunnel, both of whom were doctoral students in Kamat’s lab at the time of the study. “Lipid rafts are thought by some to play a role in the genesis of EVs, as EV membranes contain the same lipids found in lipid rafts,” Peruzzi said. “We hypothesized that if we engineered proteins to associate with lipid rafts, they may be loaded into the vesicles, allowing them to be delivered to other cells.”
The team used protein databases and lab experiments to determine that lipid raft-association is an efficient method to load protein cargo into EVs, enabling up to a stunning 240 times more protein to be loaded into vesicles.
After discovering this biophysical principle, the researchers demonstrated a practical application of the method. They engineered cells to produce a protein called a transcription factor, loaded it into EVs, and then delivered it to a cell to alter the recipient cell’s gene expression—without compromising the protein’s function upon delivery.
Kamat and Leonard said the main challenge in loading therapeutic cargo into EVs is that the producer cell and the recipient cell are often at odds with each other. In the cell producing the EV, for example, you might engineer therapeutic cargo to associate tightly to a membrane to increase the chance it moves into a soon-to-be released EV. However, this same behavior is often undesirable in a recipient cell because delivered cargo stuck to a membrane might be nonfunctional. Instead, you might want that cargo to release from the EV membrane and move to the cell’s nucleus to perform its biological function. The answer was the creation of cargo with reversible functions.
“Tools that enable reversible membrane association could be really powerful when building EV-based medicines,” Gunnels said. “Although we’re not yet sure of the precise mechanism, we see evidence of this reversibility with our approach. We were able to show that by modulating lipid-protein interactions, we could load and functionally deliver our model therapeutic cargo. Looking forward, we’re eager to use this approach to load therapeutically relevant molecules, like CRISPR gene editing systems.”
The researchers said they’re eager to try the approach with medicinal cargo for disease applications in immunotherapy and regenerative medicine.
“If we can load functional biomedicines into EVs that are engineered to only deliver those biomolecules to diseased cells, we can open the door to treating all sorts of diseases,” said Leonard, the co-corresponding author and a McCormick professor. “Because of the generalizability that we observed in our system, we think this study’s findings could be applied to deliver a wide array of therapeutic cargos for various disease states.”
