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A Targeted siRNA Screen Identifies Regulators of Cdc42 Activity at the Natural Killer Cell Immunological Synapse

The oscillating activity of a cytoskeletal regulator enables natural killer cells to effectively perform their surveillance functions and polarize cytotoxic vesicles. Oscillatory Behavior at the Immunological Synapse Natural killer (NK) cells are required for effective immune responses against virally infected cells and tumor cells. The activity of NK cells is controlled by coordinated signals from stimulatory and inhibitory receptors at the cell surface, which are engaged when the NK cell forms conjugates with target cells. Effective cell killing by NK cells is dependent on changes in the actin cytoskeleton that require Rho family GTPases, such as Cdc42. Carlin et al. used fluorescence-based live-cell imaging to show that after an initial “spike,” the activity of Cdc42 at the NK cell–target cell interface oscillated over time. Bioinformatics analysis and a short inhibitory RNA (siRNA)–based screen identified Akt and a subunit of PI3K as required for the stimulation and oscillation of Cdc42 activity, as well as for the polarization of cytotoxic vesicles, a critical step in NK cell cytotoxicity. Natural killer (NK) cells kill tumor cells and virally infected cells, and an effective NK cell response requires processes, such as motility, recognition, and directional secretion, that rely on cytoskeletal rearrangement. The Rho guanosine triphosphatase (GTPase) Cdc42 coordinates cytoskeletal reorganization downstream of many receptors. The Rho-related GTPase from plants 1 (ROP1) exhibits oscillatory activation behavior at the apical plasma membrane of growing pollen tubes; however, a similar oscillation in Rho GTPase activity has so far not been demonstrated in mammalian cells. We hypothesized that oscillations in Cdc42 activity might occur within NK cells as they interact with target cells. Through fluorescence lifetime imaging of a Cdc42 biosensor, we observed that in live NK cells forming immunological synapses with target cells, Cdc42 activity oscillated after exhibiting an initial increase. We used protein-protein interaction networks and structural databases to identify candidate proteins that controlled Cdc42 activity, leading to the design of a targeted short interfering RNA screen. The guanine nucleotide exchange factors RhoGEF6 and RhoGEF7 were necessary for Cdc42 activation within the NK cell immunological synapse. In addition, the kinase Akt and the p85α subunit of phosphoinositide 3-kinase (PI3K) were required for Cdc42 activation, the periodicity of the oscillation in Cdc42 activity, and the subsequent polarization of cytotoxic vesicles toward target cells. Given that PI3Ks are targets of tumor therapies, our findings suggest the need to monitor innate immune function during the course of targeted therapy against these enzymes.