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 (cPla(2)) nuclear shape sensing pathway, a key regulator of tissue inflammation and repair. Ca(2+) and inner nuclear membrane (INM) tension (T(INM)) are believed to mediate nucleoplasmic cPla(2) activation. The concept implies that T(INM) persists long enough to stimulate cPla(2)-INM adsorption. However, T(INM) may instead be rapidly dissipated by the contiguous endoplasmic reticulum (ER), with cPla(2)-INM adsorption reporting rather on changes in Ca(2+) 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.