Article

The integrin α6β1 modulation of PI3K and Cdc42 activities induces dynamic filopodium formation in human platelets

Institute of Microbiology and Immunology, National Yang-Ming University, Taipei, 112, Taiwan.
Journal of Biomedical Science (Impact Factor: 2.74). 01/2006; 12(6):881-98. DOI: 10.1007/s11373-005-9021-2
Source: PubMed

ABSTRACT Platelets are an ideal model for studying a rapid morphological change in response to various signal transduction systems. Morphological changes via the activation of integrin alphaIIbbeta3 in platelets have been investigated intensively. In contrast, activation via integrin alpha6beta1 is less well studied. Here, we provide the first biochemical evidence that integrins alpha6beta1 and alphaIIbbeta3 of platelets are associated with different membrane proteins. We also demonstrate that platelets activated by integrin alpha6beta1 show dynamic change by actively forming filopodia and never fully spreading over a period of more than an hour. In addition, platelets activated by integrin alpha6beta1 are different from those activated by integrin alphaIIbbeta3 in terms of cell-substrate contact and in their distribution pattern of actin, Arp2/3 and various phosphotyrosine proteins. The morphological appearance of platelets produced through integrin alpha6beta1 activation is highly dependent on PI3 kinase (PI3K) but less dependent on Src kinase. Suppression of PI3K activity in integrin alpha6beta1 activated platelets induces an increase in Cdc42 activity and more filopodium formation. However, both Cdc42 and PI3K activity are higher in platelets activated by integrin alpha6beta1 than in those activated by integrin alphaIIbbeta3. Taken together, this study demonstrates that the signals induced by integrin alpha6beta1 modulate at the level of PI3K and Cdc42 activity to allow platelets to actively form filopodia.

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    • "We also found WNT-3a to increase Cdc42 and Rac1-GTP levels during platelet activation. Since Cdc42 is thought to be required for filopodia growth [21], this may offer an explanation for the rapid, transient filopodia formation observed during platelet spreading. However, recent studies suggest that filopodia formation can also occur independantly of Cdc42 [26]. "
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    • "Like other members of the Rho GTPase family, Cdc42 influences a large array of cellular activities. Its downstream effectors include a large number of kinases which activate many signaling pathways [13] [14] as well as nonkinase proteins, such as neuronal Wiskott-Aldrich Syndrome protein (N-WASP) [15] which promotes actin nucleation. The evolutionarily conserved polarity proteins are localized into different regions of a cell to act as scaffolds for the recruitment of other protein complexes (reviewed in [16]). "
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    • "This would be observed especially when PI3K is required upstream of RhoA activation. Our previous studies have shown that RhoA plays a critical role during phagoctyic uptake of HSV-1 by primary human CF (Clement et al., 2006) and likewise, it has been suggested that PI3K is involved in integrinmediated signalling pathway that leads to the induction of filopodia (Chang et al., 2005). As shown in Fig. 3(c), pretreatment of human CF with the PI3K inhibitor significantly reduced RhoA activation, which may also be a reason why HSV-1 activity is adversely affected by the inhibitor. "
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