Fig. 4: ER–nuclear membrane network buffers TINM in response to tissue injury.
From: Endoplasmic reticulum disruption stimulates nuclear membrane mechanotransduction
a, Top: confocal maximum intensity projection of cPla2–mKate2 and luminal ER marker eGFP–KDEL in latrunculin A-pretreated zebrafish larvae after wounding under hypo-osmotic conditions (Π ~ 10 mOsm) at t = 00:01:15. Scale bar, 50 µm. Middle: magnification of eGFP–KDEL or cPla2 in individual cells within (position 1) and just outside (position 2) the wound region. Inset scale bars, 10 µm. Bottom: profile plots of cPla2–mKate2 emission of the depicted nuclei. Constitutive cPla2 translocation with ER fragmentation at the wound margin was observed in all wounded fish (n = 5). Reversible cPla2 translocation without ER fragmentation (in the second or third cell layer) in four of five fish. Time stamp, hh:mm:ss. * indicates centre of the UV-laser blast. b, The average radius of the laser wound and ER vesiculation zone from n = 7 larvae. These radii values were approximated from polygon area measurements from the dataset shown in a. c, An overview cartoon scheme outlining the approximate timing of stress responses to osmotic (ΔΠ), oxidative (ML162) or physical (laser wound) insults in vitro (black) or in vivo (red). Moderate osmotic shock (ΔΠ = 270 mOsm) causes rapid nuclear swelling, partial ER fragmentation and TINM, which is recovered by RVD. Under prolonged osmotic stress, cells enter the prelytic stage, with strong ER vesiculation and TINM, before they eventually lose plasma membrane integrity. Strong osmotic shock (ΔΠ = 303 mOsm), oxidative stress, and physical injury cause irreparable cell damage (dashed line), which directly initiates the prelytic stage. Left-column engagement of NMMT during each stage. As a death-transcendent ‘danger-signalling’ mechanism, NMMT via cPla2 is initiated before lysis and persists beyond. d, A hypothetical scheme of NMMT buffering by the ER. Middle: osmotic shock (ΔΠ) must unfold nuclear membrane invaginations before tension (T) can develop. Top: when the ER is contiguous with the nuclear membrane during homeostasis or mild stress, a compensatory lipid flow (dashed red line) partially relieves T. This amounts to less LPDs (shaded red regions/gauge) on the INM and less, or more reversible, hydrophobic cPla2–INM interactions, as observed in cultured cells exposed to moderate osmotic stress, in distal wound cells (a, position 2), or in perivascular macrophages (see ref. 43) in vivo. Bottom: when the ER is vesiculated by high and persistent Ca2+ after critical stress, there is no compensatory lipid flow to attenuate T and LPDs. This amounts to high (and more stable) cPla2–INM interactions. The respective cell is turned into a constitutive, inflammatory lipid signalling hub. This is observed, for example, in wound margin cells (a, position 1), prelytic osmotically stressed, ferroptotic cells, and post-lytic (that is, permeabilized) cell corpses.
