Researchers have more evidence that males and females are different, this time in the fluid that helps protect the cartilage in their knee joints.

They have found in the synovial fluid of this joint, clear differences in the messages cells are sending and receiving via tiny pieces of RNA, called microRNA, in males and females with the common and debilitating condition osteoarthritis.

The differences may help explain why the disease is more common in women as it points toward a more targeted way to diagnose and treat this “wear and tear” arthritis, said Dr. Sadanand Fulzele, bone biologist in the Department of Orthopaedic Surgery at the Medical College of Georgia at Augusta University.

“It’s a huge problem,” says Dr. Monte Hunter, chair of the MCG Department of Orthopaedic Surgery and a coauthor of the study in the journal Scientific Reports.

Synovial fluid is known to provide clues about joint health, so MCG researchers decided to look at what messages cells in the region were sending and receiving by looking inside traveling compartments in the fluid called exosomes, says Fulzele, corresponding author.

“What we found is there is no change in the number of exosomes, but a change in the microRNA cargo they carry,” Fulzele says.

They isolated the mostly round exosomes in discarded human synovial fluid from patients with and without osteoarthritis. They found in the males that 69 microRNAs were significantly downregulated and 45 were upregulated. In females, however there were 91 downregulated versus 53 upregulated.

Females just seemed more impacted: In total, they had more than 70 biological processes altered compared to males who had closer to 50, the researchers report.

Fulzele and Hunter suspect that the gender differences they found in exosome content helps explain gender differences in disease incidence and that estrogen was key to the differences.

Particularly in the females, they found microRNA that should be sending messages that are good for the joints, like promoting estrogen signaling and collagen–producing cells, turned off or otherwise altered.

Lower estrogen levels, like those that occur following menopause, prompt production of more cells that destroy bone. In this environ, those bone–consuming cells also tend to live longer, which can result in a net bone loss. Conversely, reduced osteoarthritis risk is considered a benefit or hormone replacement therapy.

MCG researchers’ hypothesize that estrogen plays an important role in determining which microRNAs the exosomes contain. In fact, when they used aromatase inhibitors to reduce the availability of estrogen, they found a small lineup of microRNAs decreased in number. When they treated cartilage cells from healthy females with exosomes from males and females with osteoarthritis, significantly fewer healthy cartilage cells lived after exposure to the exosomes from patients with disease. Expression of genes that make the extracellular matrix that is the framework of cartilage went down while expression of genes that promote inflammation increased.

They only found one microRNA, MiR–504–3p upregulated in both male and female osteoarthritis patients. Although it’s unclear what MiR–504–3p does, Fulzele thinks it degenerates cartilage, which is the crux of osteoarthritis. In future studies, they will use MIR–504–3p inhibitors to remove it from the equation and try to determine the function of this tiny piece of RNA.

All cells excrete exosomes as one way to communicate. They carry cargo like protein, lipids as well as microRNA, which can impact the expression and actions of many different genes. In the case of the synovial fluid, Fulzele says the exosome source is likely cells in the synovial membrane that lines the joints and produces the fluid.