Editors' ChoiceCell Biology

De-Adhesion for Mitotic Progression

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Sci. Signal.  02 Dec 2014:
Vol. 7, Issue 354, pp. ec337
DOI: 10.1126/scisignal.aaa3737

Adherent, flattened cells adopt a spherical shape as they enter mitosis. Actin stress fibers and focal adhesions (FAs) are disassembled, and actin relocalizes to the cell cortex. This change in shape is thought to facilitate proper spindle and cleavage furrow positioning and proper partitioning of cellular components. Marchesi et al. report that DEPDC1B [DEP (disheveled, Egl-10, pleckstrin) domain–containing 1B], which accumulated during the G2 phase and was degraded during mitosis, coordinates de-adhesion with mitotic entry in HeLa cells. Knocking down DEPDC1B with small interfering RNAs (siRNAs) delayed mitotic entry and, at the G2/M transition, inhibited nuclear envelope disassembly and nuclear accumulation of the mitotic driver cyclin B. DEPDC1B-knockdown (DEPDC1B-KD) cells were more motile and flatter than wild-type cells, had larger FAs (which took longer to disassemble), and contained more phosphorylated myosin light chain 2 (MLC2). Experimental manipulations that strengthened FAs increased cell spreading and inhibited the G2/M transition, whereas reducing adhesion rescued the mitotic delay in DEPDC1B-KD cells. DEPDC1B was not necessary for normal cell cycle progression in a strain of HeLa cells adapted to grow in suspension. The small guanosine triphosphatase (GTPase) RhoA activates Rho-associated protein kinase (ROCK), which, in turn, activates MLC2 to control cytoskeletal dynamics. Experiments with pharmacological inhibitors and siRNAs were consistent with DEPDC1B inhibiting RhoA-ROCK signaling to promote de-adhesion. Two-hybrid assays and proteomic analysis revealed that DEPDC1B interacted with the FA component PTPRF (protein tyrosine phosphatase, receptor type, F), which in turn interacted with RhoA and several RhoA-specific guanine nucleotide exchange factors (GEFs), including GEF-H1. Knocking down PTPRF or GEF-H1 rescued the mitotic delay in DEPDC1B-KD cells, and overexpressing GEF-H1 phenocopied knockdown of DEPDC1B. DEPDC1B competed with RhoA for binding to PTPRF, suggesting that, as DEPDC1B accumulates during G2, it displaces RhoA from PTPRF, thus inhibiting RhoA-ROCK signaling and promoting de-adhesion. In this model, DEPDC1B degradation at the onset of mitosis would re-establish RhoA activity, which is required for the stiffening of the cortical cytoskeleton that drives cell rounding. Experiments in zebrafish indicated that DEPDC1B also promoted mitotic progression in vivo by inhibiting PTPRF-dependent RhoA-ROCK signaling. The small GTPase Rap1 is also required for disassembly of FAs during mitosis, but it remains to be seen whether DEPDC1B also represses Rap1 activity to drive de-adhesion (see commentary by Garcia-Mata).

S. Marchesi, F. Montani, G. Deflorian, R. D’Antuono, A. Cuomo, S. Bologna, C. Mazzoccoli, T. Bonaldi, P. P. Di Fiore, F. Nicassio, DEPDC1B coordinates de-adhesion events and cell-cycle progression at mitosis. Dev. Cell 31, 420–433 (2014). [Online Journal]

R. Garcia-Mata, Arrested detachment: A DEPDC1B-mediated de-adhesion mitotic checkpoint. Dev. Cell 31, 387–389 (2014). [Online Journal]