Semen amyloids carried RNA into sperm, and embryos made at least 10x more protein

Amyloid fibrils, the sturdy protein strands best known from Alzheimer's and Parkinson's, appear to escort RNA from seminal fluid into sperm, scientists at Columbia's Zuckerman Institute report in Molecular Cell on October 26. When the fibrils carried RNA coding for a fluorescent protein, embryos fertilized with that sperm made at least 10 times more of the protein than embryos from sperm given the same RNA without fibrils, according to the institute.

The finding answers a quiet puzzle in reproductive biology. Seminal fluid is full of small RNAs that are thought to influence which genes switch on in a developing embryo, but nobody knew how those fragile molecules got from the fluid into the sperm.

A lead that started with HIV

The project did not begin with fertility at all. Earlier research had shown that amyloid fibrils in semen help HIV get into cells of the reproductive tract, lead author William Mallen explained. HIV carries its genes as RNA, so the lab of senior author Anthony Fitzpatrick asked whether the same fibrils might help ordinary RNA get into cells too.

Chemistry made the idea plausible. The semen amyloids carry a positive charge and RNA carries a negative one, so the two attract. Using fluorescent tags on both, Mallen saw them bind. Because amyloids are unusually durable and RNA is easily destroyed, the team suggests the fibrils may also act as a protective package.

Fibrils seen inside the cell, not just on it

The key evidence came from cryo-electron tomography, which builds 3D images from frozen samples. With the amyloids labeled with tiny gold particles, the researchers confirmed that the fibrils went inside the sperm cells rather than just sticking to their surface.

The team also measured how fast RNA entered sperm and found the fibrils greatly sped that up. Once inside, a protein called clusterin helped pull the RNA free of its amyloid carrier. Fitzpatrick called the result "wild biology" in the institute's announcement.

The release does not say which species' sperm and embryos were used in the fertilization tests, and the researchers say they do not yet know what the delivered RNA does once it is inside.

Why gene editing researchers might care

The size of the cargo is what points beyond reproduction. The fibrils moved not only small RNAs but RNA long enough to encode a whole fluorescent protein. Mallen suggested that human-derived fibrils might one day deliver gene-editing tools such as CRISPR with less risk of the immune reactions that the modified viruses used today can trigger. That remains a hypothesis, not something the study tested.

There is also a link to inheritance. Earlier studies cited by the team suggest seminal RNA can turn genes in the offspring on or off, and that some metabolic traits such as insulin resistance can pass down the paternal line in mice. If amyloids are the delivery route, they may be part of how a father's experiences reach the next generation. The next step is finding out what the shuttled RNA actually changes in the embryo.