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!
Researchers identify potentially druggable mutant p53 proteins that promote cancer growth
09.12.2016 | Cold Spring Harbor Laboratory
Plant-based substance boosts eyelash growth
09.12.2016 | Fraunhofer-Institut für Angewandte Polymerforschung IAP
Physicists of the University of Würzburg have made an astonishing discovery in a specific type of topological insulators. The effect is due to the structure of the materials used. The researchers have now published their work in the journal Science.
Topological insulators are currently the hot topic in physics according to the newspaper Neue Zürcher Zeitung. Only a few weeks ago, their importance was...
In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.
Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
09.12.2016 | Life Sciences
09.12.2016 | Ecology, The Environment and Conservation
09.12.2016 | Health and Medicine