Biomedical Engineering Reference
In-Depth Information
Figure 5
. Response of a preexisting phosphoinositide peak to a new chemoattractant gradient
(polarized sensitivity): (
a
) the active receptor concentration; (
b
) when the new gradient is
shallow, the phosphoinositide peak moves as a traveling wave towards the new steady state; (
c
)
when the new gradient is sharp, the original phosphoinositide peak collapses and a new peak is
created at the maximum of the new gradient.
simulations show that if the new chemoattractant gradient declines gradually
from its maximum in such a way that the receptors in the neighborhood of the
preexisting phosphoinositide peak sense the influence of the new gradient, the
preexisting peak moves like a traveling wave to the point at which the new che-
mogradient gradient has a maximum (Figure 5b). On the other hand, if the new
chemoattractant gradient is highly localized some distance away from the preex-
isting phosphoinositide peak such that the receptors in the neighborhood of the
preexisting peak do not sense the influence of the new chemoattractant gradient,
there is no wave motion. The preexisting peak retracts, and a new peak grows at
the maximum of the new gradient (Figure 5c). These results are consistent with
the phosphoinositide dynamics observed in experiments.
To explain the wavelike motion of the peak (Figure 5b), it is useful to ob-
serve that the steady-state membrane phosphoinositide peak formed in response
to the first gradient is "inert" everywhere except in the two thin "transition lay-
ers" surrounding the peak within which there is a sharp change in the gradient of
membrane phosphoinositides. By "inert," we mean that outside these transition
layers nothing is happening at steady state—there is neither diffusion nor syn-
thesis of membrane phosphoinositides. The transition layers, on the other hand,
are sites of intense activity even at steady state. In the upper half of a transition
layer, there is rapid synthesis of membrane phosphoinositides, which then dif-
fuse into the lower half of the transition layer, from where they are promptly
removed. The steady state is maintained by this precarious balance between syn-
thesis of membrane phosphoinositides in the upper half of the transition layers
and their removal in the lower half of the transition layers. If the balance is dis-
turbed by imposing a shallow chemoattractant gradient that increases the rate of
