Amoebas Get Social When Times Get Tough By Rezina Siddique
How single-celled social amoebae respond to chemical signals is
mechanism of signal amplification that occurred in the amoeba,
shedding light on the processes and behavior of more complex
but there was no way to test it. However, using their microfluidic
organisms, including mammals. A recent paper suggests that there
pattern generator, Levchenko’s group was able to validate the
is a mechanism by which amoeba amplify a desirable chemical
model experimentally. “Understanding the dynamics of chemotaxis
stimulus in order to self-organize and collectively migrate.
within this system can shed insight into how other multicellular
Amoeba are single-celled organisms with the capability to ag-
organisms, as well as how mammalian cells interact,” Levchenko said.
gregate to form a multi-cellular organism, and later to a fruiting
During aggregation, cyclic AMP (cAMP), a molecule that
body. Andre Levchenko, professor of biomedical engineering and
stimulates hunger, serves as chemoattractant. A starving social
affiliated faculty member of Johns Hopkins Institute for NanoBio-
amoeba secretes cAMP to attract other amoebae to it, which all
Technology, used the social amoeba, Dictyostelium discoideum,
travel towards the central amoeba. These other cells also start
because as a multicellular organism it contains cells with different
releasing cAMP in a periodic fashion in order to amplify the signal
genotypes. Levchenko’s team sought to clarify how external signals
and attract additional amoebae, creating a pulsating and wave-like
were amplified by the organism to facilitate aggregation.
signal. An individual cell ends up seeing waves of activity. This
“The way [these organisms] detect signals and move are similar to how neutrophils, a natural part of our immune system, detect
is similar to pacemaker cells in the heart, where periodic activity regulates cell behavior.
and move to the site of infection…They share the ability to migrate
In a population of cells, some cells are more sensitive while oth-
in a very directed way to get where they are needed,” said Levchen-
ers are less sensitive. This discrepancy is not visible when averaging
ko. When resources are plentiful, Levchenko’s team found that
the response over the entire population or when examining a single
Dicty are content in remain alone. But when food supplies run
representative cell. By applying the hunger stimulus to cells within
low, they gather into a multicellular slug.
their device, Levchenko’s group found that there is a large differ-
As a slug, he said, “they can move together to find a more favor-
ence across cells in a given population. Some cells did not respond
able location,” said Levchenko. The cell-cell communication that
at all, while others responded very strongly to the same stimulus.
takes place during the transition relies on chemotaxis, which is the
They also found that at higher doses, the majority of cells re-
movement towards or away from a chemical stimulus along a con-
sponded, while at lower doses, smaller numbers of cells responded.
centration gradient. This behavior is similar in mammalian cells,
This indicates that the cells that respond strongly must have some
relevant in both healthy and pathological conditions. Their results,
ability to amplify the signal.
are published in volume 5, issue 213 of Science Signaling.
Differential sensitivity in the cells helps them to organize. Ad-
Levchenko’s team developed a microfluidic pattern generating
aptation allows them to transiently suppress their sensitivity long
device that allows the user to control the environment and stimulus
enough to be able to form a multi-cellular organism. The adaptive
duration in a highly tunable way, while still being able to visualize
and amplification properties of the amoeba resemble what occurs
cells under a microscope. Historically, Levchenko explained, these
in bacterial chemotaxis. The results have implications for the study
types of experiments were done with pipettes, but with the device
of cell decision making versus commitment to behavior within cells
his group was able to perform their experiments with the dynamic
of a given tissue, or different types of cells that work together.
signaling responses consistent with the known behavior of the amoeba. Previously, a mathematical model was developed to explain the
Rezina Siddique is a Ph.D. student in Biomedical Engineering at Johns Hopkins with an M.S. in Nanoscale Science and Engineering.
Spring 2013
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