Engineers have enhanced lipid nanoparticles (LNPs), the groundbreaking technology behind COVID-19 mRNA vaccines, allowing them to not only traverse the blood-brain barrier (BBB) but also to specifically target particular cell types, such as neurons. This advancement is a significant leap toward the development of future treatments for neurological disorders like Alzheimer’s and Parkinson’s.
Engineers have enhanced lipid nanoparticles (LNPs), the groundbreaking technology behind COVID-19 mRNA vaccines, allowing them to not only traverse the blood-brain barrier (BBB) but also to specifically target particular cell types, such as neurons. This advancement is a significant leap toward the development of future treatments for neurological disorders like Alzheimer’s and Parkinson’s.
In a recent study published in Nano Letters, researchers show how peptides—short chains of amino acids—can act as precise targeting agents, enabling LNPs to deliver mRNA directly to the endothelial cells lining the brain’s blood vessels, as well as to neurons.
This is an important step forward in ensuring that mRNA is delivered to the right cell types for treating neurodegenerative diseases, as successful treatments will require the mRNA to reach specific locations. Previous research conducted by these scientists demonstrated that LNPs can cross the BBB and deliver mRNA to the brain but did not control the specific cells to which the LNPs were directed.
“Our initial publication was a proof-of-concept design for lipid nanoparticles,” explained Michael J. Mitchell, Associate Professor in Bioengineering (BE) and the senior author of the paper. “It’s like showing we could send a package from Pennsylvania to California, but we didn’t know where it would end up in California. Now, with peptides, we can direct the package to specific destinations with common features, such as every house that has a red mailbox.”
The Challenge of Accessing the Brain
The BBB is challenging to cross because its structure is designed to block virtually all harmful or foreign substances, including many medications; mRNA molecules, like many drugs, are too large to pass through. Additionally, the BBB actively removes substances it deems unsafe.
“You could inject a treatment straight into the brain or spine, but these methods are highly invasive,” stated Emily Han, a doctoral candidate in the Mitchell Lab and the primary author of the paper.
Since the BBB permits fat-soluble compounds (like alcohol and THC, which affect the brain) to enter, certain formulations of LNPs—partially constructed from the same types of fatty substances found in common oils—can penetrate into the brain.
Peptides vs. Antibodies
Until recently, most studies on targeting specific organs with LNPs focused on combining them with antibodies—large proteins that act like biological labels. “When antibodies are attached to LNPs, they may become unstable and increase in size, making it difficult for them to pass through the barrier,” noted Han.
In contrast, peptides, which consist of just dozens of amino acids as opposed to hundreds for antibodies, are significantly smaller. This smaller size not only makes it easier to attach large quantities to LNPs but also makes production more cost-effective. Additionally, peptides are much less prone to coalesce during LNP preparation or trigger undesired immune system reactions.
The idea to utilize peptides arose when Han had an unexpected encounter with a bat that entered her room, potentially exposing her to rabies. While investigating the vaccines she received against the disease, Han discovered that the rabies virus utilizes the rabies virus glycoprotein to cross the BBB. “That’s when I came across one of our most promising targeting peptides,” Han stated, which is known as RVG29, a segment of 29 amino acids from that protein.
Testing the Concept
To ensure the peptides were functioning correctly, the researchers first had to confirm they attached to the LNPs. “Our LNPs comprise a complex blend of nucleic acids, lipids, and peptides,” Han explained. “We needed to refine our quantification methods to isolate the peptides from all the other components.”
After confirming that the peptides were bound to the LNPs, the researchers then aimed to verify if the peptide-functionalized LNPs (pLNPs) actually reached their intended targets in animal models. “This is quite complex,” Han admitted, “because the brain has numerous cell types and a lot of fat that can interfere with measurements.” For over six months, Han diligently developed a method to carefully analyze brain tissue, akin to a mechanic taking apart an engine.
Future Directions
The next goal for the team is to find out what percentage of neurons need to be treated with pLNPs to significantly alleviate symptoms or cure neurological diseases. “Returning to our earlier analogy, do we need to deliver these to every house with a red mailbox or just 10% of them? Would treating 10% of neurons suffice?” Mitchell pondered.
Finding the answer to this question will be crucial for developing even more efficient delivery methods, bringing closer the possibility of mRNA-based therapies for Alzheimer’s, Parkinson’s, and other brain disorders.
This research was conducted at the University of Pennsylvania School of Engineering and Applied Science and received support from the U.S. National Institutes of Health (DP2 TR002776), Burroughs Wellcome Fund, U.S. National Science Foundation (CBET-2145491), and American Cancer Society (RSG-22-122-01-ET).
Additional co-authors include Sophia Tang, Dongyoon Kim, Amanda M. Murray, Kelsey L. Swingle, Alex G. Hamilton, Kaitlin Mrksich, Marshall S. Padilla, and Jacqueline Li from Penn Engineering, as well as Rohan Palanki from Penn Engineering and the Children’s Hospital of Philadelphia.
