Using a biochemical version of a computer chip, a team
led by Johns Hopkins researchers has solved a long-standing
mystery related to the mating habits of yeast cells.
The findings, described in the Feb. 18 Advance Online
Publication of the journal Nature, shed new light on
the way cells send and receive signals from one another and
from the environment through a process called signal
transduction that, when impaired, can lead to cancer or
other illnesses.
"Yeast is a very simple single-celled organism, but in
many respects it operates much like a human cell," said
Andre Levchenko, an assistant professor in the Department of Biomedical
Engineering at Johns Hopkins and supervisor of the
research team. "That's why it's been studied for many years
— because what we find out in yeast often holds true
for humans as well. In this study, we looked at how yeast
cells signal one another when they want to merge, engaging
in a type of mating behavior. Human cells 'talk' to one
another in a similar way, and it's important to understand
this process."
Yeast cells mate by sending out pheromone designed to
catch the attention of nearby cells of the opposite mating
type. When a prospective partner picks up this "scent," it
alters its shape and sends a projection toward the source
of the pheromone, leading to a cellular merger. This mating
process is regulated by proteins inside the cell called
mitogen-activated protein kinases, or MAPKs, through a
chain of chemical reactions.
First, sensors on the surface of a yeast cell pick up
signals that a mating partner is nearby. Then the message
is passed down toward the cell's control center, the
nucleus. The messengers that carry it to the nucleus are
MAPKs, which direct the cell's response by triggering
multiple genes. But biologists have been baffled for years
as to why two different forms of MAPKs perk up when the
mating call arrives. Only one of them, called Fus3,
appeared to be in charge of the courtship process, while
the other was thought to be moonlighting away from its main
job in another signaling pathway.
The role of the second type of MAPK was unclear, said
Saurabh Paliwal, a doctoral student in Levchenko's lab and
lead author of the Nature article. "Through experiments
with a microfluidic chip and with mathematical modeling, we
were able to learn that this second MAPK, called Kss1, does
play a crucial role. Without it, the mating process does
not proceed as smoothly."
The microfluidic chip was invented and patented by a
team that included Levchenko and Paliwal, who teamed up
with Alex Groisman, a physicist from the University of
California, San Diego. In place of the microscopic
electrical circuitry of a computer chip, their device
consists of a series of tiny channels and chambers, some 20
times smaller than the diameter of human hair. Within the
chip, computer-controlled fluid pressure and microscopic
valves allow the researchers to isolate and conduct
experiments on extremely small clusters of cells. "The
level of control we can achieve on the conditions affecting
just a few cells is unbelievable," Levchenko said. "This is
far beyond what you can do in a traditional biology lab
dish that's filled with a large colony of cells."
Using cameras attached to a microscope, the
researchers were able to view a microfluidic chip and study
the mating behavior of yeast cells in response to different
concentrations of pheromone in the presence or absence of
Kss1. They were surprised to find that this second MAPK,
thought to be relatively unimportant, actually helped the
yeast cells do a better job of finding a mate through two
distinct functions. First, it helped cells diversify their
responses at low pheromone concentrations so that only a
small fraction of cells might engage in "expensive" mating
behavior, which consumes a lot of cellular resources.
Second, in the cells that were attempting to mate, Kss1
improved the precision of finding the partner.
The researchers said their findings show the
importance of unraveling the role of multiple, apparently
redundant proteins that are often activated by the same
message passing through a cell. They also address why cells
do not get confused when they are activated by multiple
signaling messengers. Such findings may help produce
medications with fewer side effects and others that target
mutations associated with cancer.
Along with Paliwal, Groisman and Levchenko, the
co-authors of the Nature article were Pablo A. Iglesias, a
Johns Hopkins professor of electrical and computer
engineering; Zoe Hilioti, a postdoctoral fellow in
Levchenko's lab; and Kyle Campbell, of the Department of
Physics at the University of California, San Diego.
The research was supported by grants from the National
Institutes of Health and the National Science
Foundation.