Short-term transcriptomic response to plasma membrane injury

Plasma membrane repair mechanisms are activated within seconds post-injury to promote rapid membrane resealing in eukaryotic cells and prevent cell death. However, less is known about the regeneration phase that follows and how cells respond to injury in the short-term. Here, we provide a genome-wide study into the mRNA expression profile of MCF-7 breast cancer cells exposed to injury by digitonin, a mild non-ionic detergent that permeabilizes the plasma membrane. We focused on the early transcriptional signature and found a time-dependent increase in the number of differentially expressed (> twofold, P < 0.05) genes (34, 114 and 236 genes at 20-, 40- and 60-min post-injury, respectively). Pathway analysis highlighted a robust and gradual three-part transcriptional response: (1) prompt activation of immediate-early response genes, (2) activation of specific MAPK cascades and (3) induction of inflammatory and immune pathways. Therefore, plasma membrane injury triggers a rapid and strong stress and immunogenic response. Our meta-analysis suggests that this is a conserved transcriptome response to plasma membrane injury across different cell and injury types. Taken together, our study shows that injury has profound effects on the transcriptome of wounded cells in the regeneration phase (subsequent to membrane resealing), which is likely to influence cellular status and has been previously overlooked.

Restructuring of the plasma membrane upon damage by LC3-associated macropinocytosis

The plasma membrane shapes and protects the eukaryotic cell from its surroundings and is crucial for cell life. Although initial repair mechanisms to reseal injured membranes are well established, less is known about how cells restructure damaged membranes in the aftermath to restore homeostasis. Here, we show that cells respond to plasma membrane injury by activating proteins associated with macropinocytosis specifically at the damaged membrane. Subsequent to membrane resealing, cells form large macropinosomes originating from the repair site, which eventually become positive for autophagy-related LC3B protein. This process occurs independent of ULK1, ATG13, and WIPI2 but dependent on ATG7, p62, and Rubicon. Internalized macropinosomes shrink in the cytoplasm, likely by osmotic draining, and eventually fuse with lysosomes. We propose that a form of macropinocytosis coupled to noncanonical autophagy, which we term LC3-associated macropinocytosis (LAM) functions to remove damaged material from the plasma membrane and restore membrane integrity upon injury.

Interdisciplinary Synergy to Reveal Mechanisms of Annexin-Mediated Plasma Membrane Shaping and Repair

The plasma membrane surrounds every single cell and essentially shapes cell life by separating the interior from the external environment. Thus, maintenance of cell membrane integrity is essential to prevent death caused by disruption of the plasma membrane. To counteract plasma membrane injuries, eukaryotic cells have developed efficient repair tools that depend on Ca2+- and phospholipid-binding annexin proteins. Upon membrane damage, annexin family members are activated by a Ca2+ influx, enabling them to quickly bind at the damaged membrane and facilitate wound healing. Our recent studies, based on interdisciplinary research synergy across molecular cell biology, experimental membrane physics, and computational simulations show that annexins have additional biophysical functions in the repair response besides enabling membrane fusion. Annexins possess different membrane-shaping properties, allowing for a tailored response that involves rapid bending, constriction, and fusion of membrane edges for resealing. Moreover, some annexins have high affinity for highly curved membranes that appear at free edges near rupture sites, a property that might accelerate their recruitment for rapid repair. Here, we discuss the mechanisms of annexin-mediated membrane shaping and curvature sensing in the light of our interdisciplinary approach to study plasma membrane repair.

Annexins: players of single cell wound healing and regeneration

Cell life is defined by a thin 4 nm plasma membrane, which separates the interior of a cell from its environment. Thus, disruption of the plasma membrane poses a critical risk to cells, which requires immediate repair to avoid uncontrolled osmotic lysis and cell death. The initial repair response to stop the leakage usually occurs within 10–45 s and implicates Ca²⁺-activated phospholipid-binding proteins including annexins. We previously reported that annexin-induced curvature of lateral membrane around the hole plays an important role for immediate resealing of human cancer cells. Once the breach has been sealed, the cell often regenerates itself by removing the damaged membrane. This process, which also involves annexins includes excision and shedding of damaged membrane implicating the endosomal sorting complex required for transport (ESCRT) III and actin cytoskeleton remodeling. Hence, studies of cell membrane repair mechanisms should differentiate between the immediate repair response happening within seconds and the subsequent regeneration phase, which occurs in the order of minutes to hours after injury.