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usually down-regulated by the D3-PI phosphatase activity of PTEN and this
activity is essential for chemotaxis. Interestingly, pten null cells exhibit
chemotaxis defects and properties of PH domain localization similar to those
of pi3k1/2 null cells expressing myr-PI3K1 (Iijima and Devreotes, 2002;
Funamoto et al., 2002).
PTEN-GFP is localized to the plasma membrane in resting cells.
Surprisingly, PTEN delocalizes from the membrane upon chemoattractant
stimulation within a few seconds and relocalizes in 30-60 s, kinetics similar
to the localization and delocalization of PI3K (Iijima and Devreotes, 2002;
Funamoto et al., 2002). This dynamism of PTEN is a mirror image of the
translocation of PH domains and PI3Ks (Figure 17.3B). In chemotaxing
cells, PTEN localizes to the lateral and posterior parts of migrating cells
but not at the leading edge. The localization of PI3K to the leading edge
and the delocalization of PTEN from the leading edge provide a
mechanism for sharpening the PI3K signal to the leading edge, thus
restricting the localization of downstream effector pathways to the very
front of the cells.
Chemotaxis regulated by MEK kinase signalling
MAP kinase cascades are activated by cell surface receptors and regulate
various intracellular responses, such as cell growth, differentiation and cell
viability (Bokemeyer et al., 1996; Herskowitz, 1995; Levin and Errede, 1995;
Marshall, 1995; Segall et al., 1995). In Dictyostelium, two MAP kinase
cascades have been identified, one of which is required for proper chemotaxis
toward cAMP (Gaskins et al., 1994; Ma et al., 1997). Genetic deletion of the
MAP kinase ERK1 or its upstream activator MEK1 results in chemotaxis and
developmental defects (Gaskins et al., 1994; Sobko et al., 2002). ERK1 is not
activated in mek1 null cells, indicating that ERK1 and MEK1 are in the
same regulatory pathway (Ma et al., 1997; Sobko et al., 2002). Both erk1 and
mek1 null strains produce very small aggregates. In single-cell chemotaxis
assays using a micropipette to provide a directional chemoattractant gradient,
the cells are very poorly polarized and produce multiple lateral pseudopodia
simultaneously. The constitutively active form of MEK1 (MEK1
S444E,T448E
)
complements the null aggregation defect, although the aggregates arrest at the
mound stage and are unable to undergo morphogenesis. MEK1
S444A,T448A
,
which cannot be phosphorylated and thus cannot be activated, fails to rescue
the null phenotype and functions as a dominant negative form when expressed
in the wild-type cells. Fibroblasts from mek1
7/7
or mekk
7/7
mice exhibit
defects in cell migration, suggesting that the involvement of the MAP kinase
kinase pathway in migration could be evolutionarily conserved from
Dictyostelium to mammals (Giroux et al., 1999; Yujiri et al., 2000).
