Molecular basis of plasma membrane rupture in lytic cell death and its inhibition by cytoprotective agent glycine

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PROJECT SUMMARY/ABSTRACT Plasma membrane rupture (PMR) in lytic cell death—including pyroptosis, necroptosis, and post-apoptotic secondary necrosis—is a cataclysmic event that releases large-size intracellular molecules known as damage- associated molecular patterns (DAMPs), which in turn propagate the inflammatory response. Lytic cell death plays an important role in host defense against pathogen infections, but its dysregulation is also implicated in many inflammatory diseases and pathological conditions. PMR was thought to be a passive event, until a recent study identified NINJ1 to be responsible for carrying out this process. NINJ1 is a 16-kDa plasma membrane protein previously identified to be mediating cell adhesion through homotypic binding. It has two transmembrane helices and one extracellular amphipathic helix. NINJ1 undergoes oligomerization to induce PMR, and the amphipathic helix seems to play an important role in this process. However, a highly similar protein NINJ2 in the plasma membrane bearing a similar amphipathic helix does not induce PMR. To understand the molecular basis of NINJ1-oligomerization mediated PMR, we use cryogenic electron microscopy (cryoEM) to study the structures of NINJ1 and NINJ2 oligomers. The progress we have made is shedding light on a mechanistic understanding of this process. However, it also points to more hypotheses that need to be tested in order to fully understand this fundamentally important process. PMR is also linked to glycine cytoprotection in a very recent study. It was demonstrated that glycine treatment prevented NINJ1 oligomerization and thus prevented PMR in bone marrow derived macrophages receiving various forms of lytic cell death stimuli. We will include glycine treatment in our test of hypotheses for the search of signal that triggers NINJ1 oligomerization. This further elucidation of the molecular basis of glycine cytoprotection would inform the development of better cell preservation strategies or agents.