When we drink little, we produce less urine. But how is this process regulated? An international team of scientists led by Prof. Kai Schmidt-Ott of the Max Delbrück Center for Molecular Medicine (MDC) has now shed light on how the kidneys concentrate urine.
When water intake is low, humans and other higher organisms produce very small quantities of urine. “To help the body retain as much fluid as possible, water is reabsorbed from urine within the kidneys’ collecting duct system. This process is vital,” explains Prof. Kai Schmidt-Ott of the MDC and Charité’s Department of Nephrology and Medical Intensive Care.
A mouse kidney under the microscope. Cross-sections of two collecting ducts are shown.
Credit: Janett Ruffert, Kai Schmidt-Ott, MDC
For this reabsorption to work, the renal medulla that surrounds the collecting ducts must accumulate large quantities of salt and urea. Only then can water follow the osmotic gradient and be absorbed from the collecting duct into the renal medulla, where it eventually re-enters the circulation.
GRHL2 makes collecting duct cells impermeable
“For the first time, we have identified an important molecular switch that is able to maintain a high salt concentration in the renal medulla,” says the first author of the study, Dr. Christian Hinze of the MDC. This “switch” is the protein grainyhead-like 2, or GRHL2 – a transcription factor that can control the activity of genes.
The molecule is produced in the collecting duct cells and enables these cells to form a tight barrier between the urine and the medulla. Together with colleagues from Charité in Berlin and researchers from Kiel, Norway and the United States, the MDC scientists have now published their findings in the Journal of the American Society of Nephrology.
Genetically modified mice produce more urine
Experiments conducted in collecting duct cell cultures showed that GRHL2 has a significant impact on the permeability of the connection between cells. “Normally, the collecting duct cells form a tight barrier between the urine and surrounding tissue,” says Dr. Hinze. “But cells lacking GRHL2 become permeable to certain substances.” Experiments showed that collecting duct cells lacking the molecule GRHL2 become leaky and allow salts and urea to pass across cell-to-cell contacts.
Next, the scientists used an animal model to test whether these findings could be extrapolated to a living organism. They generated mice that lacked the gene encoding GRHL2 in the collecting duct system of the kidneys.
“These genetically engineered mice appeared normal and healthy at first sight,” says Schmidt-Ott. He adds that even their kidneys looked almost completely normal; only under the microscope could they see that the collecting duct cells were slightly smaller than usual. “However, the genetically altered mice produced more urine than usual, and this urine was also more dilute,” explains Hinze. Furthermore, the scientists found that the medullary region of their kidneys contained a reduced concentration of sodium.
Kidneys failed when the mice lacked water
The increased urine production became a problem as soon as the mice had limited access to water. Their creatinine and urea levels – two important laboratory indicators of kidney function – shot up drastically. “It appeared that the kidneys of these mice were failing,” says Hinze.
“This way, we were able to demonstrate for the first time how important the collecting duct cell barriers are for maintaining high concentrations of solutes in the kidney’s interstitium– and thus for regulating the concentration of urine,” adds principle investigator Schmidt-Ott. Given that the human kidney also produces GRHL2, the researchers anticipate that these results will be relevant for humans.
GRHL2 a potential target for new treatments
“What we found is fundamentally new information, which we can now use to further investigate conditions like diabetes insipidus, a severe and potentially devastating disease in humans,” says Schmidt-Ott. This disorder involves the kidneys excreting abnormally large amounts of urine, resulting in a frequent need to urinate and to drink excessive amounts of fluids. Researchers at the MDC are now interested in finding out whether GRHL2 can be controlled in order to one day offer better treatment options for patients with disorders of their water balance.
Christian Hinze et al (2018): "GRHL2 Is Required for Collecting Duct Epithelial Barrier Function and Renal Osmoregulation." J Am Soc Nephrol 29. (online 13.12.2017) doi:10.1681/ASN.2017030353
The Max Delbrück Center for Molecular Medicine (MDC)
The Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC) was founded in Berlin in 1992. It is named for the German-American physicist Max Delbrück, who was awarded the 1969 Nobel Prize in Physiology and Medicine. The MDC's mission is to study molecular mechanisms in order to understand the origins of disease and thus be able to diagnose, prevent and fight it better and more effectively. In these efforts the MDC cooperates with the Charité - Universitätsmedizin Berlin and the Berlin Institute of Health (BIH) as well as with national partners such as the German Center for Cardiovascular Research and numerous international research institutions. More than 1,600 staff and guests from nearly 60 countries work at the MDC, just under 1,300 of them in scientific research. The MDC is funded by the German Federal Ministry of Education and Research (90 percent) and the State of Berlin (10 percent), and is a member of the Helmholtz Association of German Research Centers.
https://www.mdc-berlin.de/1156685/ – Lab site of Kai Schmidt-Ott "Molecular and Translational Kidney Research"
Annette Tuffs | Max-Delbrück-Centrum für Molekulare Medizin in der Helmholtz-Gemeinschaft
NIH scientists illuminate causes of hepatitis b virus-associated acute liver failure
14.11.2018 | NIH/National Institute of Allergy and Infectious Diseases
Fish recognize their prey by electric colors
13.11.2018 | Rheinische Friedrich-Wilhelms-Universität Bonn
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
Physicists at ETH Zurich demonstrate how errors that occur during the manipulation of quantum system can be monitored and corrected on the fly
The field of quantum computation has seen tremendous progress in recent years. Bit by bit, quantum devices start to challenge conventional computers, at least...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
14.11.2018 | Life Sciences
14.11.2018 | Earth Sciences
14.11.2018 | Medical Engineering