Urinary tract infections (UTIs) are annoying and painful, and they can be fatal if left untreated. The body has a robust natural defense, however, a protein called uromodulin that can engulf invading bacteria. Researchers hope a better understanding of how uromodulin interacts with pathogens on a molecular level could lead them to new ways of preventing and treating UTIs. Now, researchers have used high-resolution imaging to reveal uromodulin’s secret: sugars.
The most abundant protein in human urine, uromodulin is produced in the kidneys, where it assembles into polymeric filaments. A team of scientists lead by Martin Pilhofer, a molecular biologist at ETH Zurich, decided to investigate the filaments’ structure. Using cryo-electron tomography—a technique that compiles high-resolution images of a sample from multiple angles—the researchers created the first 3-D model of uromodulin filaments (Science 2020, DOI: 10.1126/science.aaz9866).
The new images show a highly flexible, zigzag backbone with evenly spaced arms that branch out along its length. “This flexibility allows the filament to bind a high number of pili”—hair-like structures on the surface of bacteria, Pilhofer says. The filaments wrap the bacteria together, making it easier for the body to wash the invaders away before they cause an infection.
Uromodulin’s binding ability is largely driven by the sugars that decorate the filaments, particularly mannose, Pilhofer says. A glycosylation map the team created using mass spectrometry showed that glycans are presented all along the filament arms. They also showed that high-mannose glycans in uromodulin are integral to binding the type of pili that Escherichia coli, the leading cause of UTIs, use to adhere to their host’s cells.
Many microbes, including E. coli, use a variety of pili types to navigate their environment. Pilhofer and others on the team wanted to know if the other glycans found in uromodulin contributed to bacterial binding. They discovered that galactose and sialic acid were also involved in capturing E. coli, which “indicates that these other pili which you can find on uropathogenic E. coli are indeed recognizing the other sugars on uromodulin,” says ETH molecular biologist Gregor Weiss. Multiple glycan binding sites on E. coli could serve as novel drug targets for fighting and preventing UTIs.
When the team looked at urine samples from patients with confirmed UTIs, they were surprised to find that E. coli are not the only bacteria to get trapped by uromodulin. “In every sample where we found bacteria, they were associated with uromodulin filaments,” Weiss says. The team also observed Klebsiella pneumoniae, Pseudomonas aeruginosa, and Streptococcus mitis tangled in filaments.
Karsten Zengler, a microbiologist at the University of California San Diego, who was not involved with the study, was fascinated by the results. “It seems that the human body has found a way to use these filaments as a decoy so the bacteria swimming around don’t attach to the surface of the urinary tract but to the filament,” he says.
Now that the researchers know how these filaments capture and aggregate E. coli, they want to solve the atomic-level protein structure of uromodulin to better understand how the filaments are assembled—something that remains a mystery. This study shows the importance of human glycans in disease prevention, and “lays the foundation for a lot of future work,” says Zengler.