With assistance from the mouse genome sequence and Li Ding, Ph.D., a research instructor at the Genome Sequencing Center at Washington University, Chen and his colleagues showed that the protein mutated in the mice is aquaporin 2 (aqp2), which belongs to an important family of proteins that channel water across membranes.
"We knew that the aqp2 protein is found in the principal cells of the collecting duct, the final stretch of the kidney filtration machinery," Chen says. "Water and some other useful components are reabsorbed from the urine here. The concentrated urine is then passed on to the bladder and other downstream parts of the urinary system."
When scientists used an antibody to identify where aqp2 is in the kidneys of the mutant mice, they saw that distribution of the protein was changed. Normally aqp2 is concentrated on the side of collection duct cells that faces the urine, where aqp2 can extract water from the waste stream for recirculation in the body. In the mutant mice, though, aqp2 was scattered around the collection duct cells.
"The protein is still there, but it's not in the right place," Chen says. "To make sure this was the cause and not just a result of the problem, we analyzed the sequence of the aqp2 gene from the mutant mice, comparing it to the gene from other normal mice, and found a single base pair had changed."
The change swaps the amino acid serine for the amino acid leucine at a key position in the protein. Serine can undergo phosphorylation, a form of chemical modification frequently used in biological processes; leucine cannot. This change apparently disrupts the processes that otherwise produce a normal distribution pattern for the protein.
Chen compares the machinery that transfers water from the kidney to a plumbing system. With aqp2 unable to reabsorb water, he says
Contact: Michael C. Purdy
Washington University School of Medicine