Annexin A4 mechanically stabilizes the nuclear envelope following membrane rupture

The nuclear envelope (NE) preserves genome integrity by maintaining nuclear compartmentalization but is susceptible to rupture under mechanical stress. Although mechanisms that restore NE continuity have been identified, how damaged nuclear membranes are mechanically stabilized remains poorly understood. Here, we identify annexin A4 (ANXA4) as a Ca²⁺-dependent regulator of NE stability. Using live-cell imaging and targeted membrane injury, we show that ANXA4 rapidly accumulates at damaged regions of the NE, where its sustained association requires Ca²⁺ binding and trimer formation. Functionally, ANXA4 limits the loss of nuclear compartmentalization following injury, including under conditions of increased nuclear fragility. Atomic force microscopy of isolated nuclei reveals that ANXA4 reduces nuclear surface roughness and increases the apparent stiffness of the nuclear envelope, while experiments with reconstituted membranes demonstrate that ANXA4 directly enhances resistance to mechanical rupture. Together, these findings support a model in which ANXA4 provides an early mechanical defense against nuclear membrane damage, revealing membrane reinforcement as a protective mechanism that helps preserve nuclear compartmentalization following rupture.