Biomedical Engineering Reference
In-Depth Information
6.17e
, the cells dynamically accumulate ater approximately 2 hours in the ''dead-end” parts of
the maze. hese results show that under nutrient-deprived conditions,
E. coli
search out each
other in a collective manner, using the environmental topology of complex structures to create
traveling waves of high cell density, the irst step in quorum sensing. hrough periods of stress,
directed probing of conined structures could be vital for bacterial survival. Research on quo-
rum sensing is important for producing anti-quorum sensing antibiotics. All current antibiot-
ics aim to kill bacteria by inhibiting the synthesis of new bacteria (so natural selection rapidly
selects for resistant strains), whereas anti-quorum sensing antibiotics would disrupt the bacte-
ria's ability to communicate (and thus diminishing its ability to become pathogenic). Because
anti-quorum sensing antibiotics do not kill the bacteria, there is no survival advantage for the
resistant mutations (hence these are unlikely to occur).
6.3.4 Signal Transduction Studies Using Biomimetic Devices
In most cell biology experiments in vitro (i.e., based on cell cultures or explants), the response of
cells to various ligands is studied by bathing the cells in a homogeneous ligand solution and seed-
ing them on a homogeneous substrate. In addition to being a poor analogue of the physiological
scenario in the organism (in which cells are stimulated locally) homogeneous stimulations can
produce confounding information on how the signal is transduced from the membrane to the
cytoplasm. By stimulating the cell in only part of its membrane, some modes of transmission of
the signal can be revealed. (Particular types of cell-cell local signaling found in neural systems,
such as those involved in synapse formation, synapse transmission, and neuron-glia interac-
tions, are discussed later.)
As an example, here we take the case of how
epidermal growth factor
(EGF) signaling
spreads within a cell ater activation of its receptor (EGFR). In general, activation of the EGFRs
by EGF is signaled to the nucleus through the Ras-Raf-MEK-ERK pathway, but it also intervenes
in a variety of phenomena such as membrane traicking, cell adhesion, and cytoskeletal orga-
nization. Does EGF signaling remain localized or is it propagated in response to local stimula-
tion? Researchers at RIKEN in Japan used a simple, two-inlet and one-outlet microluidic device
(
Figure 6.18a
) to study how single COS cells respond to local EGF stimulation (using luorescent
indicators for Ras activation and tyrosine phosphorylation). One inlet carried plain cell culture
medium and the other one carried medium containing luorescent EGF, producing a hetero-
geneous laminar low as the two inlets merged into one channel (
Figure 6.18b
). Stimulation
was localized by positioning the EGF interface (visible in luorescence) halfway onto a single
COS cell (
Figure 6.18c
). In normal cells, both Ras activation and tyrosine phosphorylation sig-
nals were localized to the portion of the cell stimulated by EGF (
Figure 6.18e
)—a result that is
EGF
no EGF
Rhodamine-EGF
150 µm
a
b
c
d
20 min
10 min
1
0 min
2
3
High
4
10 µm
0
min
1
min
5
min
10
min
15
min
20
min
e
Low
FIGURE 6.18
Localized.activation.of.EGFR-dependent.pathways.using.laminar.low.streams..(From.
Asako.Sawano,.Shuichi.Takayama,.Michiyuki.Matsuda,.and.Atsushi.Miyawaki,.“Lateral.propagation.
of.EGF.signaling.after.local.stimulation.is.dependent.on.receptor.density,”.
Dev. Cell
.3,.245-.257,.
2002..Adapted.with.permission.from.Elsevier.)
