Biology Reference
In-Depth Information
The phenotype of pi3k1/2 null cells expressing myr-PI3K1 provided new
insights into the mechanisms controlling leading edge function. The analysis
of pi3k1/2 null cells demonstrated that PI3K is required for proper chemotaxis
and cell polarity but did not provide direct evidence that PI3K plays an
instructional role in controlling leading edge formation. Expression of myr-
PI3K1 in pi3k1/2 null cells leads to the formation of multiple lateral
pseudopodia when the cells are placed in a chemoattractant gradient,
indicating that activation of PI3K localized uniformly along the cell's
membrane can induce F-actin polymerization. Evidence that PI3K is activated
along the cell's periphery is provided by the localization of PH domain-
containing proteins at the sites of the multiple pseudopodia. In contrast to
other mutants that result in multiple pseudopodia such as paka null cells
(Chung et al., 2001b), PH domain-containing proteins also localize to the part
of the cell opposite the chemoattractant source (cell's 'posterior'). Interest-
ingly, these cells still move directionally towards the chemoattractant source,
although less e
ciently than wild-type cells, suggesting that these cells must
have other mechanisms by which they sense the chemoattractant gradient.
One mechanism could be PI3K3, which is still present in these cells and may
localize to the leading edge, as do PI3K1 and PI3K2. MEK1 and ERK1,
components of a MAP kinase pathway required for proper chemotaxis, also
localize to the leading edge (Ma et al., 1997; Sobko et al., 2002).
PTEN as a negative regulator of the D3-PI signalling pathway in
chemotaxis
PTEN, originally identified as a putative protein tyrosine phosphatase and a
tumour suppressor gene from human brain, breast and prostate cancer cells
(Li et al., 1997; Steck et al., 1997), is a 3'-specific phosphatidylinositol
phosphatase (Maehama and Dixon, 1998). Overexpression of PTEN blocks
the increase of PI(3,4,5)P
3
upon stimulation of insulin in glioma cells lacking
PTEN, and Akt/PKB is hyperactivated and the D3-PI levels are elevated
(Haas-Kogan et al., 1998; Maehama and Dixon, 1998; Maehama et al., 2001).
Identification of the mechanisms necessary to antagonize and down-
regulate the D3-PI signalling pathway is very important to understand the
regulatory control of chemotaxis. Recently, we and others demonstrated the
involvement of PTEN in chemotaxis by controlling the levels of D3-PI in
Dictyostelium (Iijima and Devreotes, 2002; Funamoto et al., 2002; Comer and
Parent, 2002). In the wild-type strain of Dictyostelium, the peak of PH domain
accumulation to the membrane is *6 s after stimulation, whereupon the PH
domain dissociates from the membrane within *20 s. In cells with decreased
levels of PTEN function ( pten null cells or PTEN hypomorphs), a substantial
