Robert Flaumenhaft, MD, PhD
Dr. Flaumenhaft indicated
no relevant conflicts of interest.
Gong H, Shen B, Flevaris P, et al. G
protein subunit Gα13 binds to
integrin αIIbβ3 and mediates integrin
“outside-in” signaling. Science. 2010;327:340-43.
αIIbβ3 is the dominant
platelet integrin with approximately 50 to 80,000 copies per platelet.
Activation of platelets induces a conformational change in αIIbβ3, which
promotes the binding of fibrinogen to the integrin. This process is termed
“inside-out” signaling. The binding of fibrinogen to αIIbβ3 then induces
“outside-in” signaling, which mediates cytoskeletal changes involved in
platelet spreading. Studies to determine how the binding of fibrinogen to
activated αIIbβ3 induces “outside-in” signaling have demonstrated an
association of Src family kinases, adaptor proteins, and other kinases with the
β3 cytoplasmic tail.1 Signaling mediated by Src and other associated proteins
promotes platelet spreading by mechanisms that are not completely understood.
Src also inhibits RhoA, enabling spreading by preventing RhoA-mediated
cytoskeleton retraction.2 However, the molecular details of how RhoA activity
is controlled by signaling proteins associated with the β3 cytoplasmic tail has
remained a puzzle. Gong et al., working in the laboratory of Xiaoping Du at
University of Illinois Chicago, have now elucidated an unexpected piece of this
puzzle by showing that the heterotrimeric G protein, Gα13, interacts directly
with the β3 cytoplasmic tail.
|Click image to enlarge
The authors observed that
silencing of Gα13 in platelets inhibited spreading on immobilized fibrinogen.
In addition, depletion of Gα13 abolished autophosphorylation of Src, suggesting
that Gα13 acts upstream of the kinase. Proof that Gα13 was interacting with the
β3 cytoplasmic tail was derived from immunoprecipitation studies demonstrating
an association of Gα13 and β3. Direct binding of Gα13 to the β3 cytoplasmic
tail was shown using recombinant proteins. Of note, the association of Gα13
with β3 was substantially enhanced by either platelet activation or incubation
with GTP-γ-S or AlF4- to drive the formation of GTP-loaded Gα13. The authors
identified switch region I as the domain of Gα13 responsible for binding β3.
With this knowledge, they designed a cell-penetrating synthetic peptide to
interfere with the Gα13-β3 interaction. This peptide inhibited
integrin-dependent Src phosphorylation, accelerated RhoA activation, prevented
platelet spreading, and promoted clot retraction. These studies form the basis
of a new model of dynamic regulation of RhoA during platelet spreading and
demonstrate that a heterotrimeric Gprotein can interact directly with an
integrin cytoplasmic tail.
Although it is
appreciated that signaling through G-protein coupled receptors can indirectly
influence integrin function by acting through downstream effectors, the
conventional wisdom has been that these receptor types use distinct secondary
signaling machinery. The observation that Gα13 binds directly to β3 challenges
this dogma. The implications of these findings are far-reaching. Several mouse
models demonstrate that “outside-in” signaling contributes to hemostasis and
thrombus formation following vascular injury.3,4 Targeting the Gα13-β3
interaction may be a viable strategy for antithrombotic therapy. More
compelling, however, is the possibility that G protein-integrin interactions are
not limited to platelets. Integrins and heterotrimeric G proteins are
ubiquitous proteins, and it is possible that Gα13 interacts with other
integrins in other cells. Indeed, the authors demonstrate an interaction
between Gα13 and β1. The extent to which Gα13 — and perhaps other G proteins —
bind to non-G-protein coupled receptors and contribute to downstream signaling
of these other receptor types remains to be determined. This work, however,
lays the foundation for future efforts to understand the degree to which G
protein coupling to noncanonical receptors affects signal transduction.
- Ginsberg MH, Partridge
A, Shattil SJ. Integrin
regulation. Curr Opin Cell Biol. 2005;17:509-16.
- Flevaris P, Stojanovic
A, Gong H, et al. A
molecular switch that controls cell spreading and retraction. J Cell Biol.
- Law DA, DeGuzman FR,
Heiser P, et al. Integrin
cytoplasmic tyrosine motif is required for outside-in αIIbβ3 signalling and
platelet function. Nature. 1999;401:808-11.
- Takizawa H, Nishimura S, Takayama N, et al. Lnk
regulates integrin αIIbβ3 outside-in signaling in mouse platelets, leading to
stabilization of thrombus development in vivo. J Clin Invest.
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