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 Ca(2+) 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 Ca(2+)-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 cytosol loss until the bilayer is restored might result not from vesicular fusion, but from rapid Ca(2+)-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.

Keywords: Hydrolase; Lysosome; Plasma membrane; Protease; Repair; Resealing; Sphingomyelinase.

Fig. 1

Diagram showing that addition of extracellular tracers identifies the vesicles accumulating next to wound sites as endocytic, and not an exocytic “patch”. (A) In earlier experiments performed without endocytic tracers in the medium, the numerous vesicles observed next to would sites were thought to undergo Ca²⁺-dependent homotypic fusion and form a large exocytic patch to reseal the membrane. (B) When wounding experiments were performed in the presence of BSA-gold the vesicles next to wound sites incorporated the extracellular tracer, showing that they originated by endocytosis. These endocytic vesicles subsequently merge generating larger endosomes that join the endocytic pathway, trafficking deeper into the cells and fusing with lysosomes [28].

Fig. 2

Diagram of a mechanism by which Ca²⁺-dependent annexin platforms and cross-linked cytosolic proteins could prevent cytosol loss from wounded cells. (A) Intact plasma membrane, with soluble annexin molecules in the low Ca²⁺ environment of the cytoplasm. (B) When the plasma membrane is wounded, Ca²⁺ flows in and cytosol starts to leak. (C) Cytosolic Ca²⁺ elevation recruits annexin to the inner leaflet of the plasma membrane, where it binds to exposed (PS) on the edge of the wound. (D) The membrane-associated annexin platform serves as a scaffold for the aggregation of cross-linked cytoplasmic proteins, forming a clot that stops cytosol leakage.

Fig. 3

Diagram illustrating the role of secreted lysosomal hydrolases in remodeling the plasma membrane and promoting lesion endocytosis. Upon wounding and Ca²⁺ influx, peripheral lysosomes rapidly fuse with the plasma membrane, releasing their contents extracellularly. The lysosomal enzyme acid sphingomyelinase (ASM) converts membrane sphingomyelin into ceramide, promoting membrane invagination and formation of endosomes that remove lesions and promote membrane resealing. Additional lysosomal hydrolases may regulate this process.