Background:  The membrane fusion events that initiate HIV-1 infection and promote cytopathic syncytium formation in infected cells commence with the binding of HIV envelope glycoprotein (Env) to CD4 and an appropriate co-receptor. The currently accepted model of syncytium formation suggests that HIV-induced cell fusion is independent of signaling stimulated by ligation of CD4 or coreceptor. 

Methods and Results:  Here, we show that HIV Env/co-receptor interactions activate Rac-1 GTPase and stimulate actin filament network reorganizations that are requisite components of the cell fusion process. Disrupting actin filament dynamics with jasplakinolide or latrunculin A arrested fusion at a late step occurring after formation of Env/CD4/co-receptor complexes. Time-lapse confocal microscopy of living cells revealed vigorous activity of actin-based, target cell membrane extensions at the target/Env-expressing cell interface. Expression of dominant-negative forms of actin-regulating Rho-family GTPases established that HIV Env-mediated syncytium formation relies on Rac-1, but not Cdc42 or Rho activation in target cells. Additionally, Rac activity was specifically up-regulated in a co-receptor-dependent manner in fusion reaction lysates, utilizing a co-receptor construct deficient in G-protein signaling.

Conclusions:  These results suggest that G-protein-independent signaling events regulate HIV-induced cell fusion and define a role for HIV Env/coreceptor interactions in activating cellular factors essential for syncytium formation.

Keywords: HIV; cell-cell fusion; Rac-1 GTPase