Amyloid fibrils — protein structures commonly associated with neurodegenerative diseases such as Alzheimer's and Parkinson's — may have an unexpected role in reproduction, according to a study recently published in Molecular Cell.
Researchers at Columbia University's Zuckerman Institute found that amyloid fibrils in seminal fluid can bind RNA and help transport it into sperm and embryos. The findings suggest these structures may have a previously unrecognized role in transferring genetic material during fertilization, while also raising questions about whether their properties could eventually be harnessed to deliver RNA-based therapeutics or components of gene-editing technologies such as CRISPR.
"I think we have [underestimated amyloids], and for a fairly mundane reason: amyloid was discovered in pathology," Anthony Fitzpatrick, Principal Investigator at Columbia University's Zuckerman Institute and senior author of the study, told DDN. "The cross-beta fold was defined by where it was first found, in plaques, in diseased tissue, rather than by what it does."
Although amyloids are widely studied in the context of neurodegenerative disease, functional amyloids have been identified across different organisms and biological processes. Bacteria, for example, use amyloid structures to build biofilms, melanosomes use amyloid-like scaffolds to help organize pigment production, and pituitary cells store peptide hormones in amyloid-like granules.
By understanding the role of amyloids elsewhere in the body, researchers may not only uncover previously overlooked biology, but also identify properties that could eventually be harnessed for therapeutic applications.
From HIV to sperm
This study started in an unusual place, specifically looking at human immunodeficiency virus (HIV) infection of sperm in the reproductive tract.
Previous research had identified amyloid fibrils in semen that enhance HIV infection. One such fibril-forming peptide, derived from prostatic acid phosphatase (PAP), is known as semen-derived enhancer of virus infection (SEVI). Its positively charged surface binds negatively charged viral particles, helping to concentrate them near the surfaces of cells in the reproductive tract and making it easier for the virus to enter.
Fitzpatrick, whose research focuses on amyloids, wondered whether the same structures could also transport other negatively charged molecules found in semen, such as RNA. "If these fibrils can bind and ferry a large negatively charged particle, what are they doing with the small ones?" Fitzpatrick said.
Amyloids would make excellent candidates to transport RNA to the sperm. Their fibrils are highly stable, resistant to degradation by proteases, and have an ordered structure that can provide a large surface for binding other molecules.
Semen contains a diverse collection of extracellular RNAs, including small RNA fragments that may influence gene regulation during early embryonic development. However, how these molecules reach sperm and potentially influence the developing embryo is still unclear.
What's striking is that the properties that make amyloid dangerous in the brain are precisely the properties that make it useful outside the cell
—Anthony Fitzpatrick, Columbia University
"What's striking is that the properties that make amyloid dangerous in the brain are precisely the properties that make it useful outside the cell," Fitzpatrick said. In this case, the researchers suspected that amyloids could help protect fragile RNA molecules by packaging them within a more stable structure.
Using fluorescent labels to track both molecules, the team confirmed that amyloid fibrils bind RNA. They also found that RNA does more than simply attach to existing fibrils; it helps drive their formation.
When the researchers combined PAP with RNA, the molecules first condensed into liquid droplets before assembling into amyloid fibrils. In experiments comparing fibril formation under agitation, RNA shortened the time required for fibril formation from approximately 26 hours to eight hours. Longer RNA molecules also accelerated fibril formation more quickly than shorter ones, suggesting that RNA length influences the assembly process.
"The nucleic acid is a participant in the assembly, not a passenger in it," Fitzpatrick said.
From sperm to embryo
The next question was whether RNA carried by amyloid fibrils could be transferred from sperm to the embryo.
To test this, the researchers loaded amyloid fibrils with mRNA encoding a fluorescent protein known as EGFP. They then exposed sperm to these complexes before using the sperm to fertilize mouse eggs. Mass spectrometry revealed that the embryos fertilized with sperm exposed to EGFP mRNA-bound amyloid contained at least ten times more EGFP than those fertilized with sperm that had been exposed to EGFP mRNA without amyloids.
The researchers also investigated how RNA might be released from amyloid fibrils. They found that a protein called clusterin roughly doubled the rate at which RNA dissociated from them. “RNA does dissociate on its own, so this is modulation rather than a switch, but it does suggest that release is regulated rather than purely passive,” said Fitzpatrick.
However, Fitzpatrick was clear about the limits of the findings. "We have shown that endogenous semen amyloids carry small RNAs, and that these fibrils can deliver RNA into sperm and on to the embryo," he said. "We have not yet shown that endogenous amyloid-bound RNA measurably changes embryonic gene expression."
Despite this, previous research has found that RNA in sperm can influence gene expression in offspring. These small RNA molecules may help transmit information about a father's experiences to the next generation, potentially affecting offspring development and metabolism.
Could amyloids help deliver RNA for gene editing?
As well as influencing reproduction, the researchers believe amyloid fibrils could potentially be developed into RNA delivery tools. In principle, this could include delivering the RNA components used in gene-editing systems such as CRISPR, although the team has not tested this application.
“As a delivery platform, the attractions are quite specific,” said Fitzpatrick. Their positively charged surfaces allow them to bind nucleic acids through electrostatic interactions, without requiring specific packaging signals or RNA sequences. This could make them more flexible than virus-like particles, which typically need to be engineered to carry particular genetic cargoes.
The fibrils also form from short peptides that self-assemble, potentially avoiding the need for viral vectors. In addition, molecular chaperones such as clusterin could help modulate when RNA is released from the fibrils. Fitzpatrick also suggested that using human-derived components might reduce the risk of immune reactions compared with some viral delivery systems.
However, there are significant questions to address before amyloids could become a practical delivery platform. The team has yet to establish how the fibrils enter sperm, what happens to the RNA inside cells, or whether the approach can be adapted to deliver genetic material into other cell types.
Safety will also be important. The same amyloid fibrils investigated in this study were originally identified because they enhance HIV infection, making their interactions with cells an important consideration for future applications.
For now, Fitzpatrick sees their most immediate value as a research tool in animal and ex vivo systems. "The chemistry is encouraging; the engineering, the safety work, and the ethics are all still ahead of us," he said.
