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 Ca²⁺ 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.