mTORC1 regulates cytokinesis through activation of Rho-ROCK signaling

Understanding the mechanisms by which cells coordinate their size with their ability to divide has long attracted the interest of biologists. The Target of Rapamycin (TOR) pathway is becoming increasingly recognized as a master regulator of cell size, however less is known how TOR activity might be coupled with the cell cycle. Here, we establish that mTOR complex 1 (mTORC1) promotes cytokinesis through activation of a Rho GTPase-Rho Kinase (ROCK) signaling cascade. Hyperactivation of mTORC1 signaling by depletion of any of its negative regulators: TSC1, TSC2, PTEN, or DEPTOR, induces polyploidy in a rapamycin-sensitive manner. mTORC1 hyperactivation-mediated polyploidization occurs by a prolonged, but ultimately failed attempt at abcission followed by re-fusion. Similar to the effects of ROCK2 overexpression, these mTORC1-driven aberrant cytokinesis events are accompanied by increased Rho-GTP loading, extensive plasma membrane blebbing, and increased actin-myosin contractility, all of which can be rescued by either mTORC1 or ROCK inhibition. These results provide evidence for the existence of a novel mTORC1-Rho-ROCK pathway during cytokinesis and suggest that mTORC1 might play a critical role in setting the size at which a mammalian cell divides.

💡 Research Summary

The study investigates how the mechanistic target of rapamycin complex 1 (mTORC1) integrates cell‑size information with the execution of cytokinesis. Using RNAi or CRISPR‑mediated depletion of four well‑characterized negative regulators of mTORC1—TSC1, TSC2, PTEN, and DEPTOR—the authors generated cells with chronically hyperactive mTORC1 signaling. Biochemical validation showed robust elevation of canonical mTORC1 outputs (phosphorylated S6K1 and 4EBP1). Despite normal mitotic entry and chromosome segregation, these cells displayed a striking cytokinesis defect: the intercellular bridge persisted far longer than in control cells, accompanied by intense plasma‑membrane blebbing, excessive actomyosin contractility, and ultimately a failed abscission event that culminated in re‑fusion of the daughter cells and polyploidy. Importantly, the phenotype was fully reversible by rapamycin, confirming that it required mTORC1 activity.

To uncover the downstream effector, the authors focused on the Rho‑GTPase/ROCK pathway, a major regulator of contractile ring dynamics. Pull‑down assays revealed a two‑ to three‑fold increase in GTP‑bound Rho in mTORC1‑hyperactive cells, and immunoblotting demonstrated heightened phosphorylation of ROCK2 substrates. Overexpression of ROCK2 phenocopied the mTORC1‑induced cytokinetic abnormalities, indicating that ROCK activation is sufficient to drive the observed defects. Pharmacological inhibition of ROCK with Y‑27632 restored normal Rho‑GTP levels, reduced blebbing, and rescued abscission, mirroring the effect of rapamycin. Combined treatment produced an additive rescue, suggesting that mTORC1 acts upstream of Rho‑ROCK rather than in parallel.

The authors propose a model in which mTORC1, when activated beyond a size‑dependent threshold, stimulates Rho‑GTP loading, thereby hyperactivating ROCK2. This leads to excessive cortical tension that impedes the final membrane scission step of cytokinesis. In this way, mTORC1 functions as a “size checkpoint” that couples cellular growth to division: only when a cell reaches an appropriate size does mTORC1 activity fall to a level that permits normal Rho‑ROCK signaling and successful abscission.

Beyond basic cell‑biology, the work has implications for cancer. Many tumors exhibit constitutive mTORC1 activation, and the authors show that such hyperactivation can drive polyploidization through the same Rho‑ROCK‑dependent mechanism, potentially contributing to genomic instability. Therefore, simultaneous targeting of mTORC1 and ROCK may represent a novel therapeutic strategy for cancers characterized by dysregulated growth signaling and cytokinesis failure.

In summary, this paper identifies a previously unappreciated mTORC1‑Rho‑ROCK axis that governs the mechanical aspects of cytokinesis, links cell‑size sensing to the final step of cell division, and opens new avenues for understanding and therapeutically exploiting cytokinetic defects in disease.