Connecting the dots: a centrosome-associated RAC1 signaling network links centrosome amplification with autophagy

In dividing cells, centrosomes are the primary microtubule-organizing centers and typically duplicate only once per cell division cycle, ensuring that there is just a single centrosome at each mitotic spindle pole. Centrosome amplification presents a paradox: although supernumerary centrosomes can induce chromosomal instability and promote invasive behavior, they may also impede cell proliferation by activating the Hippo-LATS pathway and the PIDDosome-p53 axis. Therefore, cells with supernumerary centrosomes must either tolerate the abnormal centrosome numbers or restore centrosome homeostasis. To identify pathways that allow cell proliferation in the presence of extra centrosomes, we conducted a genome-wide CRISPR/Cas9 screen in mouse embryonic stem cells to identify gene knock-outs that restored cell proliferation following induction of a polo-like kinase 4 (PLK4) transgene. In addition to components of known pathways that respond to supernumerary centrosomes, the screen identified a previously unrecognized Rho GTPase signaling network. Depletion of the RAC1 GTPase-activating protein ARHGAP15 increases autophagic flux, reduces the percentage of cells with supernumerary centrosomes in an ATG16L1-dependent manner, and restores proliferation of cells following PLK4 overexpression. Our findings support a model in which the centrosomal protein CEP170 promotes ARHGEF2-dependent RAC1 activation, enabling RAC1-GTP to interact with ATG16L1 and stimulate autophagy. ARHGAP15 counteracts this pathway, thereby limiting RAC1-ATG16L1 signaling and autophagy. Thus, our findings identify a mechanism that, in contrast to the Hippo-LATS and PIDDosome-p53 pathways, does not circumvent the effects of centrosome amplification by disabling cell-cycle arrest pathways but activates a corrective mechanism to reduce centrosome number.