Tom Schwarz, PhD, is a cell biologist who conducts his research in a cluttered laboratory overlooking Boston Children’s Hospital. But he likens his scientific approach to that of the great explorers of the past. “It’s like marching off into the jungle,” he says, “because you really don’t know what you’re going to find.”
Schwarz and colleagues at the F.M. Kirby Neurobiology Center have just returned from an “expedition” that could profoundly change our understanding of how the nervous system forms — and give an unexpected new role to an old standby in cell biology: the kinetochore.
It’s been known for more than a century that the kinetochore, a claw-like protein complex, plays a key role in cell division. As a cell prepares to split in two, each of its chromosomes makes a copy of itself, creating two parallel strands known as chromatids, which are joined at the midpoint. Then, as long slender fibers known as microtubules prepare to pull the two chromatids apart, kinetochores grip onto the microtubules’ ends, giving them the anchors they need to ensure that each daughter cell gets a complete set of genes.
Until now, this was believed to be the kinetochore’s one and only biological role. But in a paper published last month in Developmental Cell, Schwarz reports that the kinetochore also plays a role in the formation of synapses, the critical junctions of the nervous system.
“It’s so out of the blue to have the kinetochore getting repurposed in this way,” Schwarz says. “And now, of course, we have a lot of work to do to figure out how it’s getting used, what it’s doing.”
A laborious fishing trip
Schwarz’s discovery is an illustration of the power of a technique called “forward genetics.” Many biologists practice “reverse genetics”: They induce a mutation in a known gene and then look to see how it changes the organism, hoping in that way to discover the gene’s function. Schwarz takes the opposite tack, examining organisms with a specific defect — in this case, in the nervous system — and then working to determine its genetic cause.
“You never know what you will find, and it will take you places you didn’t think you would go,” he says of this more open-ended approach.
Schwarz’s goal was to learn more about the development of the nervous system, particularly synapses, the points where one nerve cell connects to the next. “We did this incredibly laborious and difficult fishing trip for genes that would be needed for the very earliest steps of forming a synapse – with no prejudice in advance about what we might find,” he says.
Schwarz and his colleagues built on decades of worldwide research on the genetics of the fruit fly, Drosophila melanogaster. Other scientists had already created mutant flies in which whole chunks of chromosomes were missing. Schwarz’s former postdoc Asli Oztan, PhD, mated those mutant flies and then painstakingly dissected more than a thousand of their tiny embryos in search of those with misshapen synapses. Postdoc Guoli Zhao, PhD, then set about searching for the specific genes responsible for those flaws. He found one called mis12.
Schwarz was unfamiliar with the gene, but a check of the fruit fly genome revealed that it coded for a protein in the kinetochore. “I thought there had to be a mistake,” Schwarz says. “That didn’t make any sense.”
It made no sense because, once mature, nerve cells don’t divide.“By the time that cell turns into a neuron, its cell division days are over,” Schwarz says. “So there was no reason for this mutation in the kinetochore to be causing anything to go wrong in the neuron.”
Two teams converge
As the Schwarz lab was puzzling over these findings, Arshad Desai, PhD, a biologist from the University of California at San Diego, came to Harvard Medical School to give a talk. He reported that his lab had found active kinetochore proteins in the nerve cells of the worm C. elegans. “And we realized ‘We’re not alone! Maybe we’re really right about this,’” Schwarz says.
The Schwarz and Desai labs began cooperating and ultimately published their discoveries side by side in Developmental Cell. “They had started working on the same question but coming at it from completely the opposite angle, because they were specialists in the kinetochore but didn’t know much about the nervous system,” Schwarz says. “We were coming at it from the other side, not knowing even how to spell kinetochore and saying, ‘I can’t believe that’s what’s messing up the synapses!’”
Bolstered by Desai’s findings, Schwarz’s team went on to show that when a normal mis12 gene was re-inserted into the fly’s genome, its offspring had normal synapses. Next Zhao reached out to the Drosophila community, obtaining samples of flies with mutations in other kinetochore genes. They, too, had malformed synapses. Schwarz’s conclusion: “The core kinetochore complex is getting reused somehow in synapse formation.”
Repurposing something good
The kinetochore is one of evolution’s earliest inventions — so critical to cell division that it’s shared by organisms from yeast to humans. So perhaps it’s not surprising that nature has seen fit to repurpose it.
“When nature has designed something good,” Schwarz says, “it doesn’t use it just for one task and then throw it away. It will use that to build one thing and then use it to build another. Nature is very resourceful and thrifty that way.”
Still, Schwarz wondered if this was “just a weird fruit fly and worm thing.” So he and his team went on to do experiments on human and rat nerve cells. They found that kinetochore proteins were active in both, showing that this central player in cell division also has a role in forming “your brain and mine.”
New questions about brain wiring
Many neurological disorders stem from errors in the way synapses form or function. So the revelation that the kinetochore plays a role in the wiring of the brain opens up a whole new territory for neuroscientists to explore.
“It raises a hundred new questions, and I expect that many people will be interested in working on this,” Schwarz says. “It says there’s a whole aspect of how wiring happens that we didn’t appreciate.”
A week after the publication of his paper, Schwarz was still brimming with excitement.
“There aren’t too many times when you really have that sense of, ‘Oh my gosh, this is something I really hadn’t suspected at all. This is something new,’” he said. “That’s why I get such a kick out of doing this.”
Charting a new course in neuroscience?
Since most of our neurons – and the synapses connecting them – are already formed at birth, Schwarz acknowledges his discovery is not likely to have any immediate implications for treating neurological disorders. Then again, he thought the same thing 20 years ago about his work on mitochondria, the power plants of the cell.
“Back then I sort of had to apologize for studying brain mitochondria,” Schwarz says. “People would say, ‘That’s like going to an art museum and looking at the electrical outlets.’ But now, all of a sudden, mitochondria in the brain are a hot, hot item. So I don’t want to make any rash presumptions that this won’t have clinical applications. I’ve been so wrong about that before.”
Yingzhi Ye, a visiting student in the Schwarz lab at the F.M. Kirby Neurobiology Center, was a coauthor on the paper. This work was supported by the National Institutes of Health and the IDDRC Cellular Imaging, Translational Neuroscience, and Molecular Genetics Cores.