Membrane targeting and coupling of NHE1-integrinalphaIIbbeta3-NCX1 by lipid rafts following integrin-ligand interactions trigger Ca2+ oscillations

The cyclic calcium release and uptake during calcium oscillation are thought to result from calcium-induced calcium release (CICR); however, it is unclear, especially in nonexcitable cells, how the initial calcium mobilization that triggers CICR occurs. We report here a novel mechanism, other than conventional calcium channels or the phopholipase C-inositol trisphosphate system, for initiating calcium oscillation downstream of integrin signaling. Upon integrin alphaIIbbeta3 binding to fibrinogen ligand or the disintegrin rhodostomin, sodium-proton exchanger NHE1 and sodium-calcium exchanger NCX1 are actively transported to the plasma membrane, and they become physically coupled to integrin alphaIIbbeta3. Lipid raft-dependent mechanisms modulate the membrane targeting and formation of the NHE1-integrin alphaIIbbeta3-NCX1 protein complex. NHE1 and NCX1 within such protein complex are functionally coupled, such that a local increase of sodium concentration caused by NHE1 can drive NCX1 to generate sodium efflux in exchange for calcium influx. The resulting calcium increase inside the cell can then trigger CICR as a prelude to calcium oscillation downstream of integrin alphaIIbbeta3 signaling. Fluorescence resonance energy transfer based on fluorescence lifetime measurements is employed here to monitor the intermolecular interactions among NHE1-integrin alphaIIbbeta3-NCX1, which could not be properly detected using conventional biochemical assays.