Mammalian Cells Integrate Endoplasmic Reticulum and Nuclear Envelope signals to time mitotic entry

Abstract Accurate cell division requires coordination between organelle organization and cell-cycle progression, but how architectural and functional cues from the endoplasmic reticulum (ER) and nuclear envelope (NE)—a continuous membrane network—interface with mitotic control remains unclear. Here, we demonstrate that mammalian cells integrate ER/NE structure and functions to regulate the onset and progression of mitosis. Perturbing ER function with diverse stressors causes a selective delay at the metaphase–anaphase transition, accompanied by defective spindle assembly, chromosome misalignment, and loss of coordinated ER–chromosome organization. Under these conditions, the checkpoint protein MAD1 fails to efficiently dissociate from the NE. ER stress also disrupts microtubule-organizing centers and the centriculum , an ER-derived compartment surrounding centrosomes. Restoring ER structure by expressing the shaping proteins CLIMP63(1–192) or REEP4 rescues spindle organization and mitotic progression. Conversely, transient metaphase arrest induced by partial APC/C inhibition remodels ER morphology independently of stress, and this remodeling is reversed by CLIMP63(1–192). These findings uncover a bidirectional link between ER structure function and the spindle assembly checkpoint, identifying the organelle architecture as an instructive signal that modulates mitotic timing in mammalian cell.