In the Sept. 17 online issue of Nature Genetics, Vanderbilt University Medical Center researchers report the identification of a mutation that causes severe skeletal deformities in zebrafish by shutting down a critical protein transport pathway.
The findings are surprising, said Ela Knapik, M.D., lead investigator on the study, because this pathway is thought to be so universal that a defect would prove fatal just hours after fertilization. But the mutant fish, named crusher, hatched and survived to nine days, albeit with striking skeletal abnormalities – craniofacial defects, kinked fins and shortened body.
The pathway affected by the crusher mutation is key to transporting proteins outside of the cell. All proteins are made in the endoplasmic reticulum (ER), a labyrinthine compartment just outside the cell's nucleus. Proteins are then "packaged" into transport containers called vesicles, which traverse the gelatinous cytoplasm of the cell's interior. The vesicles eventually dock with the Golgi, a structure that resembles a pancake stack and is the last major "transit station" of the cell. In the Golgi, proteins are modified into their active, final form before being shipped out to the surface of the cell in another type of vesicle. Once they reach their destination, the proteins either empty out into the extracellular space or take up residence in the cell membrane.
"Protein transport and secretion is a fundamental function of every living cell, in every organism," said Knapik, associate professor of Medicine and Cell and Developmental Biology. Similar mutations in yeast and cultured cells were lethal from the start, suggesting that no multicellular animal would be able to survive such a defect.
But, the crusher mutation appears to only affect chondrocytes, the cells that form the fish's cartilaginous skeleton. Chondrocytes secrete proteins like collagen into the extracellular space, laying down a rigid matrix (the extracellular matrix or ECM) that will form cartilage.
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 transport to a human craniofacial birth defect.
In the same issue of Nature Genetics – and back-to-back with Knapik's paper – a group of researchers from the University of California at Davis report the human variant of this gene, which causes a craniofacial condition called CLSD (Cranio-Lenticulo-Sutural Dysplasia) with strikingly similar defects to the crusher fish.
Although CLSD is a rare syndrome, there are hundreds of human congenital dysmorphologies of the skeleton, some of which might involve defects in this protein trafficking pathway. Knapik's model may provide insights into these disorders.
"No craniofacial or skeletal deformities – one of the most prominent human syndromes – had ever been linked to that pathway," Knapik said. "I'm very excited that now we have an animal model to study."
Melissa Marino | EurekAlert!
The interactome of infected neural cells reveals new therapeutic targets for Zika
23.01.2017 | D'Or Institute for Research and Education
Ion treatments for cardiac arrhythmia — Non-invasive alternative to catheter-based surgery
20.01.2017 | GSI Helmholtzzentrum für Schwerionenforschung GmbH
For the first time ever, a cloud of ultra-cold atoms has been successfully created in space on board of a sounding rocket. The MAIUS mission demonstrates that quantum optical sensors can be operated even in harsh environments like space – a prerequi-site for finding answers to the most challenging questions of fundamental physics and an important innovation driver for everyday applications.
According to Albert Einstein's Equivalence Principle, all bodies are accelerated at the same rate by the Earth's gravity, regardless of their properties. This...
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
23.01.2017 | Process Engineering
23.01.2017 | Physics and Astronomy
23.01.2017 | Life Sciences