Under a microscope, type II collagen can mainly be found in the extracellular space. Only small amounts can be seen in the cytoplasm.
In the crusher fish, Knapik and colleagues found no extracellular type II collagen in the mutant tissue. Instead, the protein was either stuck within a bloated ER or associated with the proteasome, the cell's garbage disposal. In addition, the Golgi appeared shrunken and abnormal. This suggested that the protein somehow missed the first leg of its journey out of the cell, getting stuck at the first transit station, the ER.
The researchers have identified the source of the defect ?a gene called sec23a, which is a critical component of the vesicles that transport proteins from ER to Golgi. But since the gene is supposedly active in all cells, just why chondrocytes are the only cell type affected by the mutation remains unclear.
"The fact that it affects only chondrocytes is very strange," Knapik said.
One possibility is that the fast growth of the craniofacial skeleton, which begins forming around day three, is more sensitive to the slow-down of protein transport than other cell types. Still, the results suggest that another unidentified mechanism for protein transport may exist in the other cell types.
"We had expected mutations in proteins like collagen or accessory matrix proteins to cause craniofacial malformations. Realistically, nobody suspected that these so-called 'housekeeping genes' are responsible for that sort of phenotype."
"For me, it's fascinating that the gene we have found was the least expected."
It turns out that the zebrafish mutant has a human counterpart, making the crusher mutant the first animal model that links ER to Golgi protein tra
Source:Vanderbilt University Medical Center