Transparent Fish Illuminates Formation of Ductal Network
Scientists have employed a see-through fish for their research into the molecular mechanisms underlying formation and loss of the network of ducts connecting the liver, gallbladder, pancreas, and intestine. A system of ducts carries bile from the liver to the intestine to aid in fat digestion, with some bile stored in the gallbladder. Additionally, this ductal network carries digestive enzymes from the pancreas to the intestine, where they aid in digestion and absorption of nutrients. Some congenital conditions in humans result from improper formation or loss of these ductal networks. Two recent studies of these ductal networks utilized the zebrafish—a common animal model for research due to its ease of breeding and its transparency, which allows organ systems to be visualized directly.
In one study, scientists used a genetic screen and a fluorescently labeled marker to identify factors that regulate liver development in zebrafish larvae. They found that a gene, called snapc4, was important for liver development. The snapc4 gene codes for a protein, similarly named snapc4, which regulates other genes with respect to whether they are on (expressed) or off. Livers of zebrafish with mutated (defective) snapc4 were much smaller than normal. Further investigation of these mutants showed blocked transport of lipids consumed by the fish from the intestine to the gallbladder, indicating impaired functioning of the biliary duct network. By closely monitoring development of the larvae after fertilization, they observed that the biliary duct network initially formed, but the cells then died by a process called apoptosis and disappeared later in development. Additionally, the team found that the defective snapc4 protein was impaired in its ability to bind with another protein called snapc2 in a larger functional complex, suggesting that both proteins are important for maintaining the bile duct network. The snapc4 mutant zebrafish larvae showed features similar to the human disorders known as biliary atresia and vanishing bile duct syndrome, which are marked by biliary duct destruction from the disappearance of differentiated biliary epithelial cells through apoptosis. Future research could help determine if snapc4 and snapc2 play a role in this disorder.
Another research team used the zebrafish to investigate development of the ducts connecting the liver, pancreas, and intestine. They zeroed in on a gene called sox9b, which encodes a protein, sox9b, involved in regulating gene expression (although different from the factor identified in the other zebrafish study). Sox9b itself is expressed specifically in the ductal system. While researchers had previously identified the mouse version of sox9b, it had been difficult to study because even one copy of a mutated form of this gene is lethal for mice. Zebrafish, however, survive to adulthood with two genes encoding non-functional proteins, enabling studies of the effect of this gene on ductal development. Zebrafish with the non-functional form of sox9b exhibited impairment of the ductal system early in development, with blocked bile flow, as well as cells that should become liver and pancreatic ductal cells mistakenly taking on characteristics of the wrong cell types. Using fluorescently labeled markers for specific cell types, the scientists saw that the fish with this non-functional sox9b protein displayed a malformed network of ducts connecting the liver, pancreas, and intestine. Additional experiments showed that the sox9b protein is also important in maintaining ductal cell signaling through another pathway involved in early organ development through a protein called Notch. These findings shed new light on development of the ductal system and possible candidate genes (e.g., the human SOX9 gene) underlying human conditions with similar features such as biliary atresia or Alagille syndrome.