Tilapia are interesting critters: The fish can change the workings of their gills based on the saltiness of the water they are in. In a recent Molecular & Cellular Proteomics paper, researchers looked into the molecular details of how tilapia change protein expression in their gills to accommodate different concentrations of salt.
There are four species of tilapia, which belong to the large family of cichlids, and they all easily mate with one another. The hybrids are grown in fish farms around the world. “These fish have a large economic value as a source of protein and other nutrients,” explains Dietmar Kültz at the University of California, Davis, who was the first author on the MCP paper.
Kültz says the tilapia’s ability to adapt easily to the environment has made them an invasive species. They’ve left their native Africa and are swarming into places in North America, such as Florida and Hawaii.
He also points out that projected effects of climate change include rises in sea levels and more frequent droughts. “Knowing the molecular basis of tilapia’s high environmental stress tolerance will offer insight into potential strategies for managing their aquaculture performance and invasiveness,” says Kültz. “In addition, such research reveals the mechanisms that equip fish with an extreme capacity for tolerating salinity stress. Those mechanisms will likely be under great selection pressure in many species of fish exposed to future climate changes.”
Kültz and colleagues used proteomic methods to analyze many of the proteins in the gills of the fish, the organs that take up water to extract oxygen from it. First, the researchers looked at proteins known to be involved in handling salinity. They found that the expression of mitochondrial proteins, molecular chaperones and ion transport proteins was increased as salt concentrations increased.
Next, the investigators looked for novel proteins involved in salinity processing. They discovered a protein, called NDRG1, whose expression decreased with increasing salinity. This protein never has been implicated in gill reconstruction, although it is known to be involved in cell proliferation and differentiation. The investigators suspect that NDRG1 stalls cell growth: When salinity increases and the fish need more cells in their gills to handle all the salt ions, they turn down levels of NDRG1.
Kültz explains the investigators are now interested in the mechanisms by which the protein expression levels are altered by salinity and how other organs in the fish cooperate with the gills to increase the fish’s tolerance to rising amounts of salt.