The tortuous intestinal tube interfacing the inner and outer worlds is lined with epithelial cells joined by tight junctions. These gasketlike connections form an ion-selective barrier to diffusion between the cells. But not all gaskets are created equal. Their permeability can vary with the cells that contain them, creating biological nuances with implications for a variety of disorders. A recent study, published online April 18 in the Journal of Biological Chemistry by HMS researchers, further characterizes the properties of a junction that is mutated in a genetic disorder of the kidney.
Dating back to the 1950s, physiologists noticed differences in the electrical resistance across epithelia. “It seemed that some gaskets are leakier than others,” said Daniel Goodenough, the Takeda professor of cell biology. Goodenough has studied intercellular junctions since he isolated gap junctions—the connection between one cell’s cytoplasm and another’s—as an HMS graduate student in the 1960s.
A deeper understanding of the functional implications of epithelium-specific leakiness has remained murky. But the rare genetic renal disorder in humans provided some clues. Patients with the disorder, called familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), cannot keep magnesium in their bodies. A 1999 report in Science showed that mutations in the integral membrane protein claudin-16 were responsible for the disorder, and it offered Goodenough and his research team a molecular approach to studying the protein’s role in magnesium homeostasis.
“The first conclusion was that claudin-16 is a selective magnesium channel,” Goodenough said. But preliminary expression data by Jianghui Hou, a research fellow in Goodenough’s lab, demonstrated only broad cation selectivity. As a result, he said, “Our hypothesis was that claudin is actually a sodium channel, not a magnesium channel, and that a loss of sodium permeability results in a failure to generate the driving force for magnesium reabsorption.” They were correct.
Goodenough and Hou used RNA interference to create a claudin-16–deficient mouse. The knockdown of the kidney-specific claudin-16 protein produced a mouse phenotype of the human disorder with high magnesium levels in the urine. Goodenough’s collaborator Markus Bleich, professor of physiology at the University of Kiel in Germany, measured the ion selectivity of the paracellular channels in individual kidney tubules to determine which ions were moving. Mice lacking claudin-16 showed a significant loss of cation selectivity, but were not different in their lithium, sodium, or magnesium permeabilities.
“So, claudin-16 does not simply permit paracellular diffusion of magnesium from the filtrate urine back into the body,” Goodenough said. “It actually functions to generate the electrical driving force for magnesium recovery. Who would have imagined that?”
