Zebrafish genetic research results in significant discovery for geobiologists

Scanning electron microscopy images of zebrafish inner ear otoliths depicting the changes induced upon reduction of starmaker activity. A wild-type otolith is shown in the background. The wild-type otolith and the star-shaped morphant otolith (middle image) are composed of aragonite, a form of calcium carbonate, whereas the most severely affected morphant otlith (forground) is composed of calcite. Loss of starmaker activity leads to a switch in crystal selection. Full size image available through contact

PORTLAND, Ore. -- A researcher at Oregon Health & Science University's Oregon Hearing Research Center and colleagues at the Max Planck Institute have discovered a new gene that has profound effects on the formation of inorganic crystals in the inner ears of zebrafish.

The study is featured on the cover of this week's issue of Science magazine.

The newly discovered gene starmaker encodes a protein that controls the size and formation of calcium carbonate crystals in otoliths, or "ear stones," present in the inner ears of zebrafish, the researchers report. Otoliths act as inert weights involved in the perception of gravity, and in fish are involved in the perception of sound. In human ears, biominerals are present in the form of "otoconia," or "ear dust," which also are made of calcium carbonate, but the structures are much smaller. Otoconia are important for balance in humans.

By reducing the activity of starmaker in live zebrafish, the investigators were able to change the otoliths -- normally smooth, round, stone-like structures -- into elaborate, star-shaped crystals. Changing the shape of the crystals also caused the fish to have problems with balance and orientation, according to co-principal investigator Teresa Nicolson, Ph.D., OHSU Oregon Hearing Research Center's newest recruit who conducted the work with colleagues at the Max Planck Institute for Developmental Biology in Tübingen, Germany, and the European Synchrotron Radiation Facility in Grenoble, France.

"We were looking for homologues (counterparts) in zebrafish for the human gene responsible for deafness and teeth formation," said Nicolson, assistant professor in the Oregon Hearing Research Center and scientist in OHSU's Vollum Insitute. "We identified the zebrafish gene starmaker, which, although similar to the human gene DSPP (dentin sialophosphoportein) in that it encodes a protein with many negatively-charged amino acids, has an overall structure that is different."

"This (work) represents the first example of a single protein that can select one form of calcium carbonate over another. Normally, aragonite, which is a favorite among mineral collectors in the form of pearls, is present in fish otoliths. If we completely remove the protein in fish by a genetic means, then calcite, a different form of calcium carbonate, is formed instead," Nicolson explained.

"This work should aid investigators exploring the boundary between biology and mineral formation, specifically those researching the synthesis of inorganic materials. Plus, our study gives a hint as to what is happening in human DSPP patients. The human gene is probably involved in biomineralization of teeth in a similar way that starmaker mediates crystal formation in otoliths."