interplay of PI3K and the phosphoinositide 3'-phosphatase (PTEN). This
remarkable synergy results from the spatial localization of the two enzymes
relative to the leading edge of the cell as it climbs the concentration gradient
during chemotaxis. PI3K and its product phosphatidyl inositol (3,4,5)
trisphosphate (PIP 3 ) localize to the leading edge membrane while PTEN
and its PIP3 hydrolysis activity localizes to the lateral and basal membranes
away from the gradient (Funamoto et al., 2002; Iijima and Devreotes, 2002).
This differential distribution suggests a mechanism for how the shallow
chemoattractant gradient is amplified inside the cell to permit persistent
movement up the gradient. These results also emphasize the importance of a
signal upstream of the activation of PI3K and PTEN that causes their correct
asymmetric localization. Without this initial asymmetry in signalling, the
differential localization of these enzymes would not occur and this would lead
to their activation uniformly in the cell which would collapse rather than
amplify the gradient of chemoattractant outside.
Is actin polymerization the initial asymmetry generating event?
In Dictyostelium, a very early event in signalling in response to the
chemoattractant cAMP is an early actin polymerization transient in
association with the membrane that peaks by 5 s and is not associated in
time with pseudopod extension. Pseudopod extension does not begin until
after 30 s of stimulation and is associated with a late actin polymerization
transient in time and space (Cox et al., 1992; Eddy et al., 1997). The
significance of this early actin polymerization transient may be its association
with the activation of PI3K since it shows the same time course and location
as PIP 3 accumulation in the leading edge membrane during chemotactic
stimulation with cAMP (Funamoto et al., 2002; Iijima and Devreotes, 2002;
Eddy et al., 1997). This suggests that the early actin polymerization transient
defines a region near the cell membrane as the initial asymmetry compartment
that determines the location of PI3K activation.
A similar pattern of early and late actin polymerization transients is
observed in carcinoma cells in response to chemotactic stimulation with EGF.
Here again the early transient defines the leading edge and precedes lamellipod
extension, while the late polymerization transient is spatially and temporally
coincident with lamellipodium extension (Figure 11.4) (Bailly et al., 1998;
Chan et al., 1998). In vertebrate cells, more is known about the early actin
polymerization transient and the case for its involvement in EGF signalling
has been explored.
The EGF receptor is an actin-binding protein with a profilin-like actin-
binding motif (Hartigh et al., 1992). The actin-binding domain is required for
EGF-induced tissue invasion by NIH 3T3 fibroblasts (Heyden et al., 1997).