Urinary tract infections (UTIs) can be treated by antibiotics, but may be fatal if left untreated. These infections are usually caused by what are known as uropathogenic E. coli bacteria when they bind to the cells of the bladder, ureter, or urethra with their pili. The body naturally produces a protein called uromodulin that aids in fighting UTIs. However, the exact process by which uromodulin prevents inflammation has never been understood. Now a team of researchers from ETH Zurich, the University of Zurich, and the Children’s Hospital Zurich, have discovered uromodulin’s appearance and how the protein neutralizes uropathogenic E. coli.

Their findings were published recently in the journal Science, in a paper titled “Architecture and function of human uromodulin filaments in urinary tract infections.”

“Uromodulin is the most abundant protein in human urine and forms filaments that antagonize the adhesion of uropathogens; however, filament structure and mechanism of protection remain poorly understood,” the authors wrote.

The researchers first analyzed how uromodulin binds to the bacterial pili at the molecular level. The team then used cryo-electron tomography (cryo-ET) to examine the protein.  Cryo-ET is a three-dimensional imaging technique that makes it possible to analyze the structure of complex and dynamic biological assemblies in their native conditions.

“We used cryo-electron tomography to show that the uromodulin filament consists of a zigzag-shaped backbone with laterally protruding arms,” noted the researchers. They were able to observe each link of the protein chain that contained the characteristic pattern of sugar chains to which bacterial pili like to bind.

The team also discovered that the uromodulin filaments envelop the pathogen, and that a single uromodulin filament can dock with several pili of a bacterium. “This neutralizes the pathogens,” stated Gregor Weiss, a doctoral student in molecular biology at ETH. “Once the bacteria are shielded in this way, they can no longer bind to the cells in the urinary tract, which means they can’t cause infection.” Under an optical microscope, the team also noted the formation of large clumps of hundreds of uromodulin filaments and E. coli cells, which are then presumably simply excreted with the urine.

The researchers also tested to see whether the processes they had observed in the laboratory also occur in patients. Urine samples of infected patients provided by the Children’s Hospital in Zurich were analyzed. The samples revealed the same interactions between uromodulin and the pathogens. “Without interdisciplinary collaboration between different research groups and institutes, it would have been impossible to obtain this set of findings,” stated ETH professor Martin Pilhofer, PhD, who led the electron tomography investigations.

Patients have often been given antibiotics that contain sugar mannose. To a certain extent, these prevent the E. coli bacteria from attaching themselves to the cells of the urinary tract. “Thanks to our analyses, we now know that the bacterial pili recognize not only mannose but also other sugars present on uromodulin,” said Jessica Stanisich, doctoral student and another lead author of the study. “This might indicate that treatment with combined sugar supplements would be more effective.”

The discovery also aids in the development of new active substances, explained ETH professor Rudi Glockshuber, PhD. This is due to the uropathogenic E. coli attaching themselves to the same sugar chains on the cell surfaces of the urinary tract as on uromodulin during an infection.

These results provide a framework for understanding uromodulin in urinary tract infections and in its more enigmatic roles in physiology and disease. Their findings may help to develop new strategies for the treatment of urinary tract infections in the future.

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