It's an incredibly important life process, and now researchers at Weill Medical College of Cornell University in New York City have used an exciting new technology to better understand how one key player -- a protein called clathrin -- helps regulate endocytosis like a well-oiled machine. It may also give us insights into the kinds of disease states that can happen when clathrin-regulated endocytosis goes wrong.
The findings, already published online, will appear in the April print edition of Molecular Biology of the Cell, whose editors have chosen to highlight the Weill Cornell research due to its quality and broader significance to the field.
"This is a kind of cellular fine-tuning, and we know that deficits in this fine-tuning can wreak havoc on health," said Dr. Timothy Ryan, Professor of Biochemistry at Weill Cornell Medical College. "It's well known in obesity, for example, that even a tiny change in metabolic rate results in someone becoming obese."
Endocytosis involves the shuttling back and forth of vesicles (sac-like objects) filled with nutrients, neurotransmitters, or other compounds; usually from the cell surface to the cell interior.
Previous work at Dr. Ryan's lab has helped clarify the role of some of the key players in this process, including a ubiquitous protein called clathrin, found in nearly every cell type.
"From our previous work in brain synapses, we've learned that endocytosis is a machine that really runs quite fast, and in this latest study we were trying to figure out what those speed limits are, especially with regard to varying amounts of clathrin ," Dr. Ryan explained.
However, they faced one methodological roadblock.
As Dr. Ryan explained, scientists have long known how to gauge the importance of particular proteins, using a kind of Òon/offÓ comparison. They do this by using Òknock-outÓ mice genetically engineered to either express -- or not express -- the protein under study.
But Dr. Ryan's group wanted to go deeper, and discover what happened to endocytosis when amounts of clathrin increased gradually. "The knock-out mouse model was just too blunt an instrument," he said.
Instead, they came up with a whole new method of inquiry. It combined two pre-existing technologies -- RNA interference (RNAi), where scientists cut down on protein expression by jamming the responsible gene with bits of RNA; and microscopy, which looks at the activity of individual living cells by attaching a fluorescent tag to a specific cell component -- in this case clathrin.
"This means that now we could suddenly look at a much broader dynamic range of activity," Dr. Ryan said. "Instead of it being an 'all or nothing' proposition, we were able to understand how changing different amounts of clathrin changed the speed of endocytosis."
"This had literally never been done before, which is one of the reasons the journal editors are so excited about this study," he said.
The new technology really paid off in terms of results, he added.
"It turns out that clathrin regulates endocytosis in a manner we never suspected before. It's not a linear relationship, where simply doubling the amount of clathrin doubles the speed at which vesicles are shuttled back and forth," Dr. Ryan explained. "Instead, it's what biochemists call a highly 'cooperative' relationship. That means that at a certain moment in this relationship some kind of 'tipping point' occurs, so that a relatively small increase in clathrin kicks endocytosis into high gear."
The study was conducted in fibroblasts , a kind of cell used in many cell-culture experiments. But Dr. Ryan said the findings have broad implications for cells throughout the body, including those closely tied to brain disorders, cardiac illness, and other major conditions.
He pointed out that clathrin is also important to cellular mechanisms other than endocytosis, including those occurring deep inside the cell.
"Now that we have a better handle on this relationship, we're going to go beyond looking at clathrin per se, and try and investigate other players in the endocytotic process," he said. ÒWe're going to ask questions such as, 'OK, if I tweak this molecule, how does that change clathrin's role in all of this?'Ó
"It's really quite exciting," Dr. Ryan said. "This new methodology, coupled with our better understanding of clathrin, should really help crack this field wide open."
The study was funded by grants from the National Institutes of Health and the Irma T. Hirschl Trust.
Co-researchers included Dr. Charles T. Yokoyama and Howard S. Moskowitz, both of the Department of Biochemistry at Weill Cornell Medical College.