Buffering of nuclear membrane tension and mechanotransduction by the endoplasmic reticulum revealed by quantitative ALPIN imaging

Nuclear deformation by osmotic shock or necrosis activates the cytosolic phospholipase A2 (cPla2) nuclear shape sensing pathway, a key regulator of tissue inflammation and repair. Ca²⁺ and inner nuclear membrane (INM) tension (T _INM ) are believed to mediate nucleoplasmic cPla2 activation. The concept implies that T _INM persists long enough to stimulate cPla2-INM adsorption. However, T _INM may instead be rapidly dissipated by the contiguous endoplasmic reticulum (ER), with cPla2-INM adsorption reporting rather on changes in Ca²⁺ than T _INM . The impact of T _INM and ER contiguity on nuclear shape sensing and mechanotransduction remains unknown. To address this gap, we developed the Ca ²⁺ insensitive, T _INM -only biosensor ALPIN (Amphipathic Lipid Packing sensor domain Inside the Nucleus). By quantitative ALPIN imaging, we found that stress-induced ER fragmentation increases T _INM and nuclear membrane mechanotransduction in osmotically shocked or ferroptotic cells, permeabilized cell corpses, and at zebrafish wounds in vivo. Our findings reveal critical roles for the ER and T _INM in nuclear shape sensing and introduce ALPIN as promising tool for studying organelle membrane mechanotransduction in health and disease.