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 (TINM) are believed to mediate nucleoplasmic cPla2 activation. The concept implies that TINM persists long enough to stimulate cPla2-INM adsorption. However, TINM may instead be rapidly dissipated by the contiguous endoplasmic reticulum (ER), with cPla2-INM adsorption reporting rather on changes in Ca2+ than TINM. The impact of TINM and ER contiguity on nuclear shape sensing and mechanotransduction remains unknown. To address this gap, we developed the Ca2+ insensitive, TINM-only biosensor ALPIN (Amphipathic Lipid Packing sensor domain Inside the Nucleus). By quantitative ALPIN imaging, we found that stress-induced ER fragmentation increases TINM 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 TINM in nuclear shape sensing and introduce ALPIN as promising tool for studying organelle membrane mechanotransduction in health and disease.