October 27, 2006

‘Discovery research’ yields surprising find

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Mark Magnuson, M.D., and his team are studying a protein’s role in mouse development.
Photo by Dean Dixon

‘Discovery research’ yields surprising find

Mark Magnuson, M.D., and his colleagues were simply following a hunch eight years ago when they decided to study a gene they had identified in mouse tumor cells. The gene encoded a protein that showed similarity only to a protein in slime mold.

“We were intrigued by this protein that had absolutely no defined function whatsoever, and that had no conserved domains to suggest function in mammalian cells,” said Magnuson, director of the Vanderbilt Center for Stem Cell Biology.

This month, the researchers reported in Developmental Cell that the protein, called rictor, plays a fundamentally important role in cell growth and metabolism. It is a component of a protein complex that regulates a central cell signaling molecule, and mice without rictor die during fetal development, Magnuson's group found.

The investigators weren't looking for such a protein back in the late 1990s. They were looking for genes involved in the development or function of insulin-producing pancreatic beta cells. To identify such genes (and their resulting proteins), they engineered mice to make pancreatic tumors — both insulinomas (insulin-producing tumors) and non-insulinomas.

Then they looked for genes that were “turned on” in one type of tumor and “turned off” in the other, and vice versa. Of the genes they identified using this screening strategy, several caught their attention — one of these was the gene that would later come to be known as rictor.

At the time, they knew only that their unknown gene was similar to a slime mold gene called Pianissimo. The slime mold life cycle includes production of a stalk and fruiting body, and without Pianissimo, slime mold could not complete this part of its life cycle.

“We didn't know what that meant — that a protein that was enriched in insulinomas was involved in the slime mold life cycle — but it looked like it must be doing something important,” Magnuson said.

The investigators devised a strategy for “knocking out” — deleting — the gene in mice. Their approach also allowed them to study when and where the gene is normally “turned on” and to make tissue-specific knockouts, deletions of the gene only in the pancreas, for example. Their “multiallelic” gene targeting strategy is unique, Magnuson said.

As the team worked toward the knockout mouse, other groups of scientists also identified the rictor gene, which turned out to be identical to the Pianissimo-type gene Magnuson's group was studying. Rictor, along with several other proteins, most notably one called mTOR, regulates the protein kinase Akt, a central cell signaling molecule.

Although mice without rictor die about midway through embryonic development, Magnuson and colleagues were able to use cells from these mice to evaluate cell signaling and Akt activity.

The paper offers “genetic proof,” Magnuson said, that a protein complex including rictor is responsible for regulating Akt through a process called phosphorylation. Active Akt participates in numerous cellular signaling processes, including those related to cell division, cell death, protein synthesis, and cytoskeletal organization. Both Akt and mTOR also have been implicated in tumor development, Magnuson said.

Magnuson's team is interested in Akt's function in the pancreatic beta cell, where it is involved in regulating both the number and size of beta cells, he said.

“Understanding the regulation of this molecule is really central, and these rictor knockout mice now give us a tool for being able to explore these cellular processes.”

Magnuson emphasized the thrill of this type of “discovery research” that takes investigators in new and unexpected directions.

“There was a point in this project where we really had no idea where it would lead us, and we had to take some huge chances,” Magnuson said. “We went after this gene just because it looked interesting in slime mold, and in the end we came up with a finding that is central to our understanding of cell function, both in diabetes and cancer.”

Co-authors of the paper include Chiyo Shiota, Jeong-Taek Woo, Jill Lindner and Kathy Shelton. The research was supported by the National Institutes of Health and the American Diabetes Association.

Magnuson is the Earl W. Sutherland Jr. Professor of Molecular Physiology & Biophysics.