Engineers target neurons with modified lipid nanoparticles

In a new development at the University of Pennsylvania, engineers have successfully modified lipid nanoparticles (LNPs) to not only cross the blood-brain barrier (BBB) but also to precisely target neurons. This innovation, published in Nano Letters, represents a significant leap toward mRNA-based therapies for neurological diseases such as Alzheimer’s disease and Parkinson’s disease.

Lipid nanoparticles, the technology behind the COVID-19 mRNA vaccines, are notoriously effective at delivering mRNA to cells. However, crossing the BBB – a protective barrier that shields the brain from harmful substances – has long been a challenge. Penn researchers have overcome this obstacle by using peptides, short amino acid chains, as targeting agents. These peptides allow LNPs to specifically deliver mRNA to the endothelial cells lining the brain’s blood vessels and the neurons themselves, paving the way for tailored treatments for neurodegenerative conditions.

mRNA delivery

Previous research from the team showed that LNPs could successfully cross the BBB and deliver mRNA to the brain, but the question remained: where would the mRNA go once it was in the brain? “Our first paper was a proof-of-concept lipid nanoparticle design,” explains Michael J. Mitchell, Associate Professor in Bioengineering and the study’s senior author. “It was like showing we could send a package from Pennsylvania to California, but we had no idea where in California it would end up. Now, with peptides, we can address the package to specific destinations with shared features, like every house with a red mailbox.”

This new approach gives researchers control over the destination of the mRNA, a crucial factor when developing treatments for complex neurological diseases, which require targeting specific brain cells.

Overcoming the blood-brain barrier

The BBB, a natural defence mechanism, presents a formidable challenge for drug delivery. This barrier blocks most foreign molecules, including the large mRNA molecules used in modern vaccines, from entering the brain. The structure of the BBB actively expels substances it deems harmful, making it difficult for pharmaceutical treatments to reach their target.

“You can inject a treatment directly into the brain or spine, but these are highly invasive procedures,” says Emily Han, a doctoral student in Mitchell’s lab and the paper’s first author. However, LNPs, which are composed of lipids – fat-soluble molecules – can bypass the BBB, much like alcohol or THC, substances that also affect the brain.

Why peptides?

Until now, most research aimed at targeting specific organs with LNPs has relied on antibodies. However, antibodies tend to increase the size of LNPs, making them harder to pass through the BBB. “When you put antibodies onto LNPs, they could become unstable and larger in size, which makes it really hard to squeeze through the barrier,” says Han.

In contrast, peptides are much smaller, typically comprising just dozens of amino acids. This smaller size makes them easier to attach to LNPs, while also being more cost-effective to produce. Moreover, peptides are less likely to cause aggregation during the formulation of LNPs, and they provoke fewer unintended immune responses.

To confirm that the peptides worked as intended, the team had to ensure they adhered to the LNPs. “Our LNPs are a complex mixture of nucleic acids, lipids and peptides,” Han explains. “We had to optimise quantification methods to pick out the peptides against all those other signals.” The team then tested whether the peptide-functionalised LNPs (pLNPs) could target specific cells in animal models. Given the complexity of brain tissue, which contains a variety of cell types and fats that interfere with measurements, this was no small feat. The team’s persistence paid off, and they succeeded in delivering mRNA to the brain’s endothelial cells and neurons.

Looking ahead

The next step for the team is to determine how much of the brain needs to be treated with pLNPs to effectively alleviate symptoms or even cure neurodegenerative diseases. “Returning to the same analogy, do we need to send these to every house with a red mailbox, or just 10 percent of them? Would 10 percent of neurons be enough?” asks Mitchell.

Understanding the precise amount of neuronal targeting required will be key to developing more efficient delivery strategies, bringing mRNA-based treatments for Alzheimer’s, Parkinson’s, and other neurological diseases closer to reality.

The study was conducted at the University of Pennsylvania School of Engineering and Applied Science and supported by the US National Institutes of Health, the Burroughs Wellcome Fund, the US National Science Foundation, and the American Cancer Society.

This study was published in Nano Letters.