Biomedical Engineering Reference
In-Depth Information
2. The role of PIP
3
remains elusive. It has been shown, for instance, that
PI3K, the enzyme that catalyzes the synthesis of PIP
3
, is sufficient, but not nec-
essary for polarization. Delivery of exogenous PIP
3
to neutrophils provokes po-
larity development (9). However, in
Dictyostelium
cells lacking both PI3K1
and PI3K2 show only partial defects in chemotaxis (49).
3. The influence on polarization of molecules that are downstream of PIP
3
is controversial. In
Dictyostelium
, phosphoinositides are polarized even if actin
polymerization is inhibited by latrunculin (27). This implies that phosphoinosi-
tides are polarized exclusively by reactions upstream of actin polymerization.
In neutrophils, on the other hand, the polarization is almost completely abol-
ished when actin polymerization is inhibited by latrunculin or promoted by jas-
plakinolide (33).
4. Several potential global inhibitors have been hypothesized, but the evi-
dence supporting their inhibitory role is either lacking or inconclusive. The en-
zyme PTEN, which catalyzes dephosphorylation of PIP
3
to PIP
2
, migrates in a
manner that strongly suggests that it may be a global inhibitor. While PIP
3
lo-
calizes to the "front" of the cell, PTEN translocates to the "back" of the cell
(50,51). However, the mechanism of this translocation is unknown. We have
proposed that the cytosolic pool of inositol phosphates plays the role of global
inhibitor (39). This hypothesis is supported by experiments which show that
blocking the synthesis of inositol phosphates stimulates chemotaxis (53). How-
ever, the inhibitory mechanism appears to be different from the one hypothe-
sized by us. Indeed, Luo et al. (52) have shown that in
Dictyostelium
the
inositol phosphate InsP7 competitively inhibits the binding of various PH-
domain-containing effector enzymes to PIP
3
, thus retarding their recruitment to
the membrane. Finally, it has also been suggested that cGMP fulfills the role of
inhibitor (37), but this hypotheses has not been rigorously tested.
The resolution of these outstanding issues will play a crucial role in foster-
ing further model development. Indeed, all the existing gradient sensing models
are "lumped" because our ignorance regarding the identity of the activator and
the inhibitor argues against the development of more detailed models. As noted
in this volume (see Part III, chapter 1.4, by Dhar and Tomita), a model is only as
good as the data it purports to explain. At present, we have reliable data on the
spatiotemporal dynamics of the phosphoinositide localization. By establishing
that these nontrivial dynamics are consistent with the activator-inhibitor model,
we hope that we have sharpened the focus of the experimental search for the
variables. To the extent that the activator-inhibitor model is a valid representa-
tion of the gradient sensing mechanism, the experimentalist can now focus atten-
tion on variables possessing the dynamic properties of the activator and
inhibitor. Further refinement of the model must await the identification of these
variables.
