Above the fray: Surface remodeling by secreted lysosomal enzymes leads to endocytosis-mediated plasma membrane repair

Abstract

The study of plasma membrane repair is coming of age. Mirroring human adolescence, the field shows at the same time signs of maturity and significant uncertainty, confusion and skepticism. Here we discuss concepts that emerged from experimental data over the years, some of which are solidly established while others are still subject to different interpretations. The firmly established concepts include the critical requirement for Ca2+ in wound repair, and the role of rapid exocytosis of intracellular vesicles. Lysosomes are being increasingly recognized as the major vesicles involved in injury-induced exocytosis in many cell types, as a growing number of laboratories detect markers for these organelles on the cell surface and lysosomal hydrolases in the supernatant of wounded cells. The more recent observation of massive endocytosis following Ca2+-triggered exocytosis initially came as a surprise, but this finding is also being increasingly reported by different groups, shifting the discussion to the mechanisms by which endocytosis promotes repair, and whether it operates or not in parallel with the shedding of membrane blebs. We discuss how the abundant intracellular vesicles that undergo homotypic fusion close to wound sites, previously interpreted as exocytic membrane patches, actually acquire extracellular tracers demonstrating their endocytic origin. We also suggest that an initial, temporary patch that prevents cyto-sol loss until the bilayer is restored might result not from vesicular fusion, but from rapid Ca2+-dependent crosslinking and aggregation of cytosolic proteins. Finally, we propose that cell surface remodeling, orchestrated by the extracellular release of lysosomal hydrolases and perhaps also cytosolic molecules, may represent a key aspect of the plasma membrane repair mechanism that has received little attention so far.

Self-repairing cells: How single cells heal membrane ruptures and restore lost structures

“…Sudden change in the intracellular environment could thus serve as a proxy to alert the cell to a membrane rupture. Specifically, influx of calcium ions is often a key trigger for wound response (6, 16, 17). Physiologically, this makes sense given Ca 2+ is an important intracellular messenger and underlies many important signaling pathways.…”

“…It is possible to rationalize such differences by considering the parameters in the mass transport approximations above. For example, a low extracellular Ca 2+ level would slow down the diffusive influx and subsequent accumulation of Ca 2+ to a level necessary to trigger wound response (17). Similarly, a high cellular viscosity would decrease the leakage rate of cell mass.…”

In vitro and ex vivo strategies for intracellular delivery

“…Inundation with reactive oxygen species and other toxic molecules may cause further damage to endogenous proteins and biomolecules 49 . Thus, cells urgently deploy repair pathways to reseal membrane disruptions and recover from the damage imposed (see recent reviews [70][71][72][73][74] ).…”

“…There is a marked consistency in the properties of the repair pathways, regardless of whether the source of damage is electrical, mechanical, optical, or even chemical 70,74 . Instead, membrane recovery is thought to depend on disruption size, collateral damage, temperature, composition of the extracellular medium, and cell type.…”

“…3b). First, Ca 2+ -dependent exocytosis and fusion of membraneproximal vesicles leads to resealing by patch formation 75 , but may also serve to reduce tension around the wound, or release lysosomal signals for membrane remodelling 74 . Second and third, for smaller disruptions of several hundred nanometres or less, endocytosis or exocytosis, respectively, may extract the lesion into a disposable vesicle.…”