Psychotherapy may prevent mental breakdowns, but a family of proteins named cadherins seems to keep the tissues in our bodies from undergoing their own form of dissociation. But until now, no one knew precisely how two cadherins on the surfaces of adjacent cells could stick together.
With X-ray crystallography, Columbia's Dr. Lawrence Shapiro has been able to understand the linkage of two cadherin molecules using crystals of the protein as narrow as a human hair and a particle accelerator a mile wide. Dr. Shapiro is associate professor of ophthalmologic science (in ophthalmology) and biochemistry and molecular biophysics, and a member of Naomi Berrie Diabetes Center. Based on his findings about the cadherin connection, pharmaceutical companies are developing potential anti-angiogenesis drugs against blood vessel cadherins. The work, supported in part by the National Institutes of Health, was published May 17 in Science.
Cadherins come in 30 different varieties and each prefers to stick to its own kind. For example, an N-cadherin only sticks to another N-cadherin. Cadherin specificity is quite useful in development to keep the cells of one tissue together, and to keep different tissues apart.
But without seeing how an N-cadherin binds to an N-cadherin, or how a nearly identical E-cadherin binds to E-cadherin, it was hard to tell why N-cadherins rarely stick to E-cadherins and the 28 other classical cadherins.
To get a picture of two identical cadherins caught in the act of adhesion, Dr. Shapiro used X-ray crystallography. In his first attempt as a graduate student in the 1990s with Dr. Wayne Hendrickson, University Professor of Biochemistry and Molecular Biophysics, the cadherin crystals he made revealed two cadherins intimately entwined with each other. But the orientation of the two proteins showed that both would have to be in the same cell to bind together. The more interesting questionhow cadherins on different cells stick togetherremained unclear.
Two years ago, as an assistant professor at Mount Sinai, Dr. Shapiro made more crystals of C-cadherin and took them to the particle accelerator at Brookhaven National Laboratory on Long Island. There, he froze the crystals in liquid nitrogen and suspended one crystal from a fiber. As the crystal slowly rotated, extremely bright X-rays generated from the particle accelerator blasted the crystal.
Just as a crystal chandelier scatters rays of colored light all over the dining room walls, the cadherin crystals dispersed the incoming X-rays. Cameras captured the action and now the researchers had a 3-D map of spots, each spot corresponding to a scattered, or diffracted, X-ray beam. Because the strength and location of diffracted beams depends on the molecule's shape, computers could analyze the spot data and extract C-cadherin's form.
This time, the crystal showed two cadherins in a more revealing position. The tip of each boomerang-shaped molecule sent out a flexible arm to the other. The end of each arm fit into a dent in the adjacent molecule, forming a molecular "ball-and-socket" joint. Using the arms, the two cadherins can link two different cells.
However, the new picture of cadherin adhesion still does not answer why only identical cadherins stick to each other. Dr. Shapiro says the site of adhesion might be too small to contain enough information to distinguish an N-cadherin from the 29 other cadherins. "The specificity of cadherins is more complex than we thought," Dr. Shapiro says. "It's not just a one-on-one code."
For drug design, the picture reveals opportunities for preventing cadherin adhesion and possibly stopping cancer growth. Many growing cancers need new blood vessels to provide the cancer with oxygen and nutrients. Cells that gather together to form new blood vessels use VE-cadherin to stick to each other and form the vascular tube. Dr. Shapiro is working with two pharmaceutical companies that used his cadherin picture to design drugs to block VE-cadherin adhesion. Blocking adhesion could disrupt new blood vessel growth, and potentially starve the cancer.
Dr. Shapiro's next X-ray crystallography target is the structure of a different class of cadherins that glue neurons together. He also researches Tubby and other genes that may have a role in adult obesity. Ophthalmology, biochemistry, and the diabetes center recruited him to return to Columbia a year ago. "I'm thrilled to be back at Columbia," Dr. Shapiro says. "I've been to a lot of places and I think Columbia is the best intellectual environment I've encountered."