Measurement of plasma membrane rupture via confocal microscopy

To maintain homeostasis, cells undergo a tightly regulated process called programmed cell death. Some forms of programmed cell death, such as pyroptosis, can elicit a strong inflammatory response by releasing cytokines through small protein pores. The terminal event of pyroptosis in most cells is plasma membrane rupture (PMR), which breaks down large sections of the plasma membrane and emits intracellular contents that can further amplify the inflammatory signal. In opposition to the previous dogma that PMR is a passive event, it was recently discovered that the transmembrane protein, Ninjurin-1 (NINJ1), is the key executor of PMR. Two models of NINJ1-mediated PMR predict that NINJ1 oligomerizes into filaments or ring-like structures to either open large pores or to excise sections of the membrane. Both models underpin how NINJ1 must oligomerize to execute PMR. When at rest, NINJ1 will autoinhibit oligomerization and activation by forming face-to-face dimer-dimer structures on the plasma membrane. Follow-up studies have shown that NINJ1 executes PMR for other forms of programmed cell death, including apoptosis, ferroptosis, and PANoptosis, as well as mechanical cell death. Thus, assessing NINJ1 function directly through quantification of NINJ1 oligomerization is important to expanding our understanding of both programmed and mechanistic cell death. Here, we describe methods to visualize and quantify NINJ1 oligomerization via immunofluorescence imaging of NINJ1 puncta. This protocol enables more precise and accurate measurement of NINJ1 function during PMR, surpassing conventional methods that just quantify PMR by-products.