"We were excited by the idea that physical adhesion between microvilli might provide the driving force for growing and tightly packing the brush border," Tyska said.
The investigators suspected that proteins in the cadherin family calcium-dependent adhesion molecules that allow cells to stick together might mediate the interaction between microvilli. They had previously used proteomics technologies to define all of the proteins in the isolated brush border. The list included two candidates for microvillar tip adhesion: protocadherin-24 and mucin-like protocadherin.
In a series of studies, the team demonstrated that these two cadherins have a role in sticking microvilli together. Experimental reduction of protocadherin-24 in the cell culture model destroyed the brush border. Microvilli still formed, but they were not tightly packed and they had variable lengths.
"It's always been a question how microvilli achieve this remarkably uniform length," Tyska said. "Now it looks like the solution is simple they get tied together at the tips and one can't get past another. It's really straightforward."
In biochemical experiments, the researchers discovered that the cadherin molecules interacted with two proteins inside microvilli a motor protein connected to the cell cytoskeleton and an adapter protein called harmonin. These connections were required for the localization of the cadherins to the microvillar tips.
Interestingly, harmonin had been studied by other investigators for its role in linking protrusions on the inner ear hair cell a sensory structure involved in hearing. Genetic mutations that disable harmonin cause Usher syndrome, a form of inherited deaf-blindness. Patients with Usher syndrome also have poorly characterized intestinal disease.
Tyska and his colleagues realized that harmonin mutations might disrupt the intestina
|Contact: Leigh MacMillan|
Vanderbilt University Medical Center