Actin Cytoskeletal Reorganizations and Coreceptor-Mediated Activation of Rac during Human Immunodeficiency Virus-Induced Cell Fusion

Abstract

The membrane fusion events which initiate human immunodeficiency virus type 1 (HIV-1) infection and promote cytopathic syncytium formation in infected cells commence with the binding of the HIV envelope glycoprotein (Env) to CD4 and an appropriate coreceptor. Here, we show that HIV Env-coreceptor interactions activate Rac-1 GTPase and stimulate the 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 in the formation of Env-CD4-coreceptor complexes. Time-lapse confocal microscopy of living cells revealed vigorous activity of actin-based, target cell membrane extensions at the target cell-Envexpressing cell interface. The expression of dominant-negative forms of actin-regulating Rho-family GTPases established that HIV Env-mediated syncytium formation relies on Rac-1 but not on Cdc42 or Rho activation in target cells. Similar dependencies were found when cell fusion was induced by Env expressed on viral or cellular membranes. Additionally, Rac activity was specifically upregulated in a coreceptor-dependent manner in fusion reaction cell lysates. These results define a role for HIV Env-coreceptor interactions in activating the cellular factors essential for virus-cell and cell-cell fusion and provide evidence for the participation of pertussis toxin-insensitive signaling pathways in HIV-induced membrane fusion.

Distinct Requirements for HIV-Cell Fusion and HIV-mediated Cell-Cell Fusion

“…46 for the opposite result). By comparison, actin rearrangement induced in target cells upon contact with Env-expressing cells (26,47) or by HIV-1 particles sandwiched between target cells could augment the pore dilation through generating a lateral force within the PM.…”

Tetraspanins CD9 and CD81 Modulate HIV-1-Induced Membrane Fusion

“…Another possibility is that CD81 alters actin organization in viral synapses through the regulation of integrin function (15). Actin remodeling is essential for the fusion process (57)(58)(59) and is required for the recruitment of membrane molecules into clusters where fusion events take place. We and others (3,60,61) have observed a ring of actin at viral synapses, and drugs that disrupt actin network inhibit membrane fusion.…”

Moesin is required for HIV-1-induced CD4-CXCR4 interaction, F-actin redistribution, membrane fusion and viral infection in lymphocytes

“…F-actin reassembly appears to be involved in Env-induced CD4, CXCR4 and LFA-1 redistribution to cell-cell junctions during viral spreading (Jolly et al, 2004), and drugs that affect actin dynamics appear to inhibit Env-mediated fusion (Eitzen, 2003;Pontow et al, 2004); but it is unclear whether these drugs affect HIV-1 Env activity (Campbell et al, 2004;Yonezawa et al, 2005). Although there is a clear correlation between HIV-1-mediated recruitment, CD4 and co-receptor polarization, and efficient viral fusion and infection (Iyengar et al, 1998;Jolly et al, 2004;Manes et al, 2000), little is known about the molecular mechanisms regulating the CD4-co-receptor interaction.…”