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
Colorectal cancer risk factors decrypted
13.07.2018 | Max-Planck-Institut für Stoffwechselforschung
Algae Have Land Genes
13.07.2018 | Julius-Maximilians-Universität Würzburg
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
13.07.2018 | Event News
13.07.2018 | Materials Sciences
13.07.2018 | Life Sciences