HIV-1-infected T cells can form stable conjugates with non-infected T cells in a process known as “virological
synapse” formation. This process is reminiscent of the formation of an “immunological synapse”, during which
CD4+ T cells rapidly polarize the actin cytoskeleton, the microtubule-organizing center (MTOC), and cytokine-
containing vesicles towards antigen-presenting cells. The polarization of CD4+ T cells during immunological
synapse formation depends on the RHO family GTPase CDC42, a molecular switch that has a key role in the
establishment of polarity in eukaryotic cells.
We have now observed that CDC42 is critical for the efficient spreading of HIV-1 in several T cell lines
and in primary cells. However, our data also imply that CDC42 is dispensable for the completion of a single
cycle of replication. Together, our observations implicate CDC42 in the cell-to-cell transmission of HIV-1.
CDC42 stimulates the formation of membrane extensions, such as filopodia, through effectors that
mediate the polarization of the actin cytoskeleton, and HIV-1 can exploit filopodial bridges to spread from cell
to cell. Thus, our data let us to propose a working model in which CDC42 is crucial for the formation of
intercellular extensions that facilitate the transfer of HIV-1 between CD4+ T cells. In support of this model, we
have observed that CDC42 is required for the formation of HIV-1-induced membrane extensions by MOLT-3
cells. An alternative working model is that CDC42 is required for the polarized trafficking of HIV-1 virion
components to the virologic synapse.
We propose to directly examine the roles of CDC42 in HIV-1 cell-to-cell transmission and virological
synapse formation, and to determine whether HIV-1 regulates the activity of CDC42. We also propose to
examine the roles of CDC42 effectors that regulate localized actin assembly and polarized trafficking in HIV-1
spreading. Among these effectors are F-BAR proteins that connect to actin polymerization machinery, as does
the F-BAR protein PACSIN2, which we have recently implicated in the cell-to-cell transmission of HIV-1.
Notably, our preliminary results indicate that certain CDC42 effectors, including the CDC42-regulated actin
polymerase FMNL1 and a putative CDC42 effector that controls polarized exocytosis, have crucial roles in
The proposed studies have the potential to yield fundamental new insights into the mechanism of an
important but poorly understood mode of HIV-1 transmission. Of particular significance would be the
identification of a kinase downstream of CDC42 as being critical for HIV-1 spreading, since protein kinases
constitute one of the most important groups of drug targets.