Volume 2, Issue 9
Ring Around the Collar for Chromosomes
Cells have amazing quality control records when it comes to passing on genetic material during cell division. Only once every 100,000 times or so do the chromosomes containing the DNA misalign during division. Even then, the cell usually catches the problem and halts the process. UC Berkeley professor David Drubin and his colleagues hope to understand the secret to this precision by taking a very close look at the microscopic machines in the cellular factory.
The research team: (from left) Eva Nogales, Stefan Westermann, David Drubin and Georjana Barnes (courtesy Berkeley Lab)
"When a chromosome becomes misaligned, it can lead to birth defects or cancer," says Drubin, a professor of molecular and cell biology. "But it doesn't happen very often. One of my research thrusts is to try to understand the cellular mechanisms that ensure this high fidelity."
For a cell to divide properly, it must first duplicate its chromosomes. The pairs of chromosomes remain connected at the center in a structure called a kinetochore. Then, the cytoskeleton that gives shape to the cell breaks down and the microtubule fibers that formed that cytoskeleton transform into a spindle. Next, the chromosomes line up along the ends of the microtubule fibers and a biomolecular process literally pulls the chromosome pairs apart. As they travel to opposite ends of the spindle, the microtubule fibers disperse before eventually forming the cytoskeleton for the daughter cells.
"The process is almost like the videogame Pac Man," Drubin says. "As the chromosomes move down the microtubule, the fiber segments just fall off."
The question though is how the kinetochore binds to the microtubule spindle to keep the chromosomes segregated before they split apart. Recently, Drubin—collaborating with his wife Georgjana Barnes, also a professor of genetics and development, Eva Nogales, a professor of molecular and cell biology and a staff scientist at Lawrence Berkeley National Laboratory, and postdoctoral researcher Stefan Westermann—found the answer. It's a case of ring around the collar.
In this electron microscope image, rings are visibly bound to the microtubules. (courtesy the researchers)
"It's common in scientific papers for researchers to draw cartoons of the kinetochore structure so that it looks like a collar around the microtubule," Drubin says. "A collar structure would mean that segments of the microtubule could fall away but the chromosome could hold on as it moves down the fiber to the pole."
The researchers were surprised when experiments seemed to show that the cartoon art really does imitate life. In a recent study, Drubin and his colleagues expressed the kinetochore proteins using a genetically-engineered E. coli bacteria. They then mixed the proteins with microtubules in a test tube. Nogales, an electron microscopy expert, and her research group photographed the reaction.
"They immediately saw beautiful rings decorating the microtubules," Drubin says. "When they showed us the images, we almost fell on the floor."
Kinetochore proteins bind to a microtubule spindle to keep chromosomes segregated during cell division. This segregation is critical for preventing mistakes that can lead to cancer and birth defects. (courtesy the researchers)
Through those experiments, Drubin explains, Westermann and Nogales's team also demonstrated that the rings remain at the end of the microtubule, even as the fiber "peels back like a banana." Eventually, the ring, with the chromosome attached, makes its way to the appropriate daughter cell.
The next step is to seek out the rings in their natural habitat, using a high-voltage electron microscope to peer inside a living cell. Of course, the ring formation is just one of the key mechanisms in cell division. Drubin's dream is to understand the finely-tuned operation of the entire system, including how the ring grabs onto the chromosome and what cellular signal kicks off the process.
"So far, we have purely biochemical results," he says. "But when we're studying self-organizing systems like this, we'd really like to watch these processes as they happen inside living cells."
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Rearing Rodents for Behavioral Insights
Rats carry a lot of baggage. Grumpy, anti-social rats often have bad childhoods to blame. In this way, humans and rats have a lot in common. Our early life obviously has a huge affect on who we become as adults. But what is the biology behind our behavior? To find out, UC Berkeley biopsychologist Darlene Francis studies how rodents are reared.
In 2001, Darlene Francis received the Young Investigator Award from the Cure Autism Now Foundation.
"You may not care about what rats and mice are doing, but you can generalize what we learn from them to people," says Francis, who also holds a faculty position in the School of Public Health and an appointment in the Helen Wills Neuroscience Institute.
Francis joined the UC Berkeley faculty in January after several years in the Center for Neuroscience at Emory University. Her research career was interrupted by a stint as a youth counselor for children, where she became increasingly curious about how early life affects human development. Animal models enabled her to gather data and conduct controlled experiments that are simply not possible in clinical populations.
"You're never going to put a kid in a crappy environment just to see what happens," she says. "But you can ask the same questions of rats and mice. What goes so right or wrong so early in life? Can you affect change?"
One thrust of Francis's research reveals what it means to be a good mother and how differing parenting environments (both before and after birth) affect the stress levels of offspring. In a recent experiment, she took identical mouse embryos and "adopted them out" either before birth, after birth, or both, to foster moms who provided varying levels of maternal care. Once the baby mice reached adulthood, they were tested for behavioral characteristics like anxiety, ability to learn, and stress. The study showed that both the intrauterine environment and the postnatal care cooperatively influenced the mice's adult behavior.
Rats may hold valuable clues about how biology affects human behavior.
Along with conducting behavioral tests, Francis and her colleagues can gain a more direct view of the animals' neurobiology. By measuring the hormones the rodents produce or ultimately sacrificing them to directly examine their brain chemistry, Francis is able to decipher how subtle variations in neurobiology are expressed in life.
"In people, you're presented with a behavior and you have to deal with it," Francis says. "In animals though, we can set them on a path and look at the effects."
While the Human Genome Project has enabled scientists to speed up the search for genes linked to specific behaviors, Francis points out that "we're more similar than we are different."
"This line of research shows that it's not necessarily about the genes you have but about how those genes get regulated," she says.
One way to regulate the expression of genes is through intervention. In a laboratory study, Francis demonstrated that the behavior of rats raised in a challenging or stressful environment can be markedly improved by stimulation or social enrichment later in their lives. Not only does a rat's behavior change based on an environmental shift, but the interventions work by altering the animal's biology. In the end, Francis's rodent research supports the old maxim that life really is what you make it.
"The ongoing nature versus nurture debate is dead," she says. "We now have the neurobiological tools to really begin to understand the interaction between the two."
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Nanoscience Imitating Nature
Matthew Francis is also part of the Synthetic Biology Department at the Lawrence Berkeley National Laboratory.
It's tough to build things that are 100,000 times smaller than the diameter of a human hair. Biology has had a few billion years to perfect the craft of building from the bottom up. That's why UC Berkeley nanoscientist Matthew Francis collaborates closely with Mother Nature. Francis and his research group use organic chemistry to assemble nanoscale devices with unprecedented capabilities that could revolutionize cancer treatment or lead to the development of highly efficient solar cells.
"Our goal is to address a big challenge in nanoscience, which is how to position objects with exquisite resolution so that the exciting components people are developing can be combined into functional devices," Francis says.
In recent years, he explains, materials scientists have developed a wide array of impressive building blocks for nanoscale systems, from computer components fashioned from single molecules to promising drug delivery systems. The problem though is constructing useful devices from these materials. Some of the new nanoparticles are too small for current lithographic techniques like those used to fabricate integrated circuits. Others are a bit oversized for precise "bottom up" positioning using organic chemistry.
Empty MS2 capsids like these could be used to house an anticancer agent in a drug delivery system.
"Biology has an enormous number of proteins that self-assemble into structures with feature sizes that are at exactly the right length scales," Francis says. "So we can use proteins as positioning scaffolds to place these interesting components into functional arrays."
For example, the researchers have transformed the shell of a bacteriophage called MS2 into a capsule that could deliver anti-cancer drugs only to tumor cells. First, the group developed a method to remove the viral genome so that an anti-cancer drug can be stored inside. Once the drug was in place, the viral shell could then be coated with a polymer that protects the cargo from triggering unwanted side effects or degrading prematurely as it travels through the body to the tumor site. Finally, the linkage attaching the drug might be functionalized to release the anticancer agent only when the nanocapsule reaches the tumor.
The tube-like capsid of the tobacco mosaic virus provides a promising template for the construction of a bioartificial light harvesting system.
The first step though, Francis says, is developing the tools of organic chemistry.
"Our specialty is designing reactions that can link the nanoparticles and organic molecules to the proteins to make these new hybrid structures."
One of his favorite examples of nature's own nanoengineering, he says, is the photosynthetic light harvesting system that enables plants and certain bacteria to convert sunlight into organic energy. In plants, an array of molecules called chromophores form a "collector antenna" that gathers the photons from the sunlight and transfers the energy to a central engine that does the energy conversion.
Research groups have already synthesized artificial chromophores with tunable optical characteristics. The goal is to devise solar cells that convert sunlight into electricity far more efficiently than today's technology. Artificial or not though, these kinds of chromophore-based light harvesting systems only function properly if each molecule is approximately three to five nanometers away from its neighbor. How can scientists possibly achieve such a precise pattern? One answer, of course, is to go back to nature.
In recent experiments, Francis and his research group have functionalized the tobacco mosaic virus, harmless to humans, so that its shell can be used as a template for the self-assembly of a wide-variety of nanomaterials. Indeed, Francis says, the rod- and disc-like shapes of the obtained assemblies are particularly well suited as a chassis for a light harvesting system inspired by nature.
"For me, the real excitement of nanoscience is that it allows you to build devices with functions that simply don't exist now and can only occur at the nanoscale," he adds.
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Berkeley's Scientific Legacy
William Wallace Campbell (1862-1938), astronomer and administrator
In 1915, William Wallace Campbell was elected president of the American Association for the Advancement of Science.
Astronomers remember William Wallace Campbell for his pioneering use of the spectrograph to measure the velocity of stars in the line of sight and for his strong leadership as Director of the Lick Observatory. In University of California history, Campbell is more widely known as a strong and compassionate president, whose administration from 1923 to 1930 was characterized by a steady growth and rising enrollment that continued even when the on-set of the Depression foreshadowed a curtailment of physical development on the campus.
Campbell graduated from the University of Michigan in 1886 with a degree in engineering. While he was a student at Michigan he worked as an assistant at the Observatory. That experience changed his life; he became fascinated by astronomy. Upon graduation, Campbell became a professor of mathematics at the University of Colorado. Two years later he was given the opportunity to return to the University of Michigan to teach astronomy -- but at half the salary he received at Colorado. It was an easy decision; he went to Michigan.
In 1890, Campbell spent the summer as a volunteer at the world-class Lick Observatory on Mt. Hamilton. There, under the tutelage of James E. Keeler, he studied the "New Astronomy," which we now know as astrophysics. Campbell was an apt student. When Keeler left Lick in 1891 to become Director of the Allegheny Observatory, Campbell was appointed to the Lick staff as Keeler's replacement. Seven years later, in 1890, Keeler returned to Mt. Hamilton as Director of the Observatory. In 1900, Keeler very suddenly and unexpectedly died. A group of the world's leading astronomers recommended that Campbell be Keeler's replacement.
Campbell in 1893 at the Lick Observatory
Over the next 30 years at Lick, Campbell encouraged and supported seminal spectrographic studies of a wide variety of astronomical objects. Frequently, the instruments used in the investigations were designed and fabricated on Mt. Hamilton. Campbell established an observing station in Chile to study stars in the southern hemisphere. He and his colleagues produced the most extensive and accurate catalog of radial velocities available then and for several decades thereafter. Campbell planned an expedition to Australia to observe the solar eclipse of 1922. The purpose of the observations was to test Einstein's theory that light passing near a massive object such as the Sun is bent by the gravitational field of the object. As an aside, in 2005 physicists worldwide are celebrating the centennial of Einstein's "miracle year" of publications, one of which predicted the light deflection. The eclipse observations, which were made and analyzed by Lick Astronomer R.J. Trumpler who later became a Professor of Astronomy at Berkeley, showed conclusively that light passing next to the edge of the Sun is bent by the amount Einstein predicted. Still, Campbell considered his greatest discovery to be that nearly one-third of the brighter stars are spectroscopic binaries; that is, systems in which two stars are so close together that they appear to be a single star, but which are shown spectroscopically to be two (or sometimes more) stars moving in orbits around their center of mass.
In 1922, Campbell accepted an invitation by the Regents to become President of the University. His scientific mindset guided his decisions as an administrator.
"The fundamental purpose of universities," he said in his inaugural address, "is to hasten the day when all men and all women shall have comprehension of the truth, so that they may live their lives more richly and more usefully in this exceedingly interesting world. The first... obligation of a university is to instruct the students who come knocking at its doors; to disseminate ... the knowledge that has been gained and preserved in all past time. The second great function of a modern university is to extend the frontiers of knowledge into regions as yet unexplored."
William Wallace Campbell died on June 14, 1938. Campbell Hall, named in his honor, houses UC Berkeley's Astronomy Department.