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to the PM via adaptor modules that bind phosphorylated
active receptors to in turn enhance the activity of the bound
PTPs by phosphorylation. The system of RTKs interacting
with different PTPs is an example where the inversion of an
interaction from activating to inhibiting changes the global
activity pattern from silenced but excitable to local activity
hotspots. If, however, we track the localization of a PTP in
response to its recruitment to the PM, the activity pattern
becomes a time-dependent transient phenomenon
(
Figure 17.2
). Before ligand binding, the system remains
stable and activatable. Weak random activity fluctuations of
the RTKs are countered by the cytosolic PTPs. Ligand
binding constitutes a strong fluctuation as it continuously
activates those RTKs to which the ligand is bound. As the
system is a balance waiting to be tipped, this perturbation
spreads more rapidly than diffusion of activated RTKs from
this point of origin would allow. This is mediated by the
inactivation of PTPs in close proximity to the PM via short-
ranged action of ROS. The diffusion-limited translocation
of a PTP to the PM by RTK activity-triggered binding to
phosphotyrosines via an adaptor module again increases
the strength of PTP activity in the globally activated state of
RTK. As a result, in regions where fluctuations reduce RTK
activity slightly, PTP activity further reduces RTK activity
and hence ROS production. The system is still able to
maintain hotspots of activity because the PM now has
a pool of inactive, diffusing RTKs that can be locally
reactivated. To test this hypothetical system and to put
constraints on the physicochemical parameters of the
modeling, experimental observations of the involved
components are obviously crucial. However, reasonable,
simple considerations about the biochemical properties of
RTKs and PTPs and the interdependence of their activities
allow the exploration of the possible manifestations of the
system on the micrometer scale.
SPATIOTEMPORAL QUANTIFICATION
OF CELLULAR PROCESSES
The macroscopic activity and localization patterns that give
rise to cellular function emerge from the interaction and
mobility of nanometer-sized molecules. These local
molecular properties are themselves modulated by the global
cellular patterns that they generate. Such simultaneous
upward and downward causation is typical for self-organized
systems. To understand the principles and mechanisms by
which a cell operates, it is therefore necessary to quantify the
progression of processes starting from a certain state with
spatial and temporal resolution at the molecular level
without losing sight of the cellular context.
Owing to its ability to resolve micrometer-sized struc-
tures, biological microscopy has been instrumental in the
discovery and understanding of living systems. Microscopy
as an extension of our eyes has quickly become a window
into the subcellular world. At the dawn of microscopy,
Robert Hooke, in 1665, published the first observation of
the basic unit of life: the cell. One of the oldest preserved
drawings is from the prolific microscopist Antonie van
Leeuwenhoek (1632
1723), who observed a lumen in the
red blood cells of salmon. The discovery of the nucleus
showed that compartmentalization does not end at
e
the
cellular level but is a pattern that repeats within itself.
As staining agents with better contrast became available
more than a century later, chromosomes, which strongly
adsorbed basophilic dyes, were identified inside the nucleus
by Walther Flemming and named by von Waldeyer-Hartz.
FIGURE 17.2
Cellular automaton simulation of an
RTK-PTP-interaction in a 3D virtual cell, depicted in
the simplified reaction schemes on top. Time progresses
from left to right column, with a coloring table for indi-
vidual pictures as last column. Top row: vertical section of
the cell; normalized RTK activity; Middle row: top view of
the basal membrane; Both: dark
e
no activity, bright
e
high
activity. Bottom row: vertical section of the cell; normal-
ized PTP concentration; dark
e
no concentration, bright
e
high concentration. Continuous PTP activity suppresses
RTK activity, which remains below an activation threshold.
A ligand-binding event perturbs the system above activa-
tion threshold. High RTK activity lowers PTP activity
proximal to the PM, allowing the autocatalytic RTK acti-
vation to spread. Although initially localized, activation
signal promptly spans the whole PM. High RTK activation
also triggers PM binding of PTP via adaptor protein (lower
row). This increases PTP concentration by sequestering PTP activity in close proximity to the RTK that raises RTK activation threshold. The PTP
translocation transforms the double-negative feedback of RTK
e
PTP interaction to an activator
e
inhibitor topology. There, RTK activity in regions with
slightly lower RTK activity is suppressed by the faster-diffusing PTP tethered to the membrane. A stable Turing pattern forms from this lateral inhibition.
This is seen most easily in the middle row, where the RTK activity declines everywhere (white to yellow to blue) apart from the local hotspots. Here local
RTK activity is sufficiently high to maintain its activity state, because the autocatalytic activation outperforms deactivation by PTP.
RTK
RTK
PTP
PTP
max
active RTK
active RTK
in basal PM
active PTP
0
time
