**3. Early events of single-cell wound healing are mechanically driven processes**

Single-cell repair proper is an active process, requiring dynamic and concerted manipulations of the cell's membrane and cytoskeletal compartments. Most of these processes, however, take place relatively late following injury or are dependent on preliminary disruption of cytoskeletal structures. In this subchapter, we present the principal wound mitigation events that are activated in the moments immediately following injury and facilitate the subsequent exocytosis-, endocytosis- or membrane-shedding-mediated wound-healing processes.

### **3.1. Caveolae-mediated decrease of in-plane membrane tension**

Caveolae are plasma membrane invaginations with a diameter of 50–80 nm and specific flasklike morphology [36]. Caveolae have long been known to flatten in response to mechanically induced membrane deformation stretch [37]. Indeed, the preincubation of cells with methylβ-cyclodextrin diminishes the time to cell lysis upon hypotonic challenge [38]. Methyl-βcyclodextrin is a cholesterol-depleting compound that has also been shown to severely reduce the number of caveolae at the cell surface [39], probably by limiting the recruitment of caveolin oligomers to the plasma membrane. Caveolae can thus be viewed as a "membrane buffer" that limits injury-induced increases in apparent membrane tension by diminishing the in-plane tension without the need of additional membrane components from Ca2+-dependent exocytosis (**Figure 1**). Instead, additional membrane area is produced by the rapid flattening and disassembling of caveolae upon mechanical stress, which are rapidly reassembled upon mechanical stress release [40].

The exact molecular events leading to caveolae assembly and disassembly is still somewhat unclear, the specifics of which go beyond the scope of this chapter. Briefly, their assembly is initiated by the clustering and further recruitment of phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylserine (PS) and cholesterol with caveolin oligomers. Recruitment of various cavins oligomers will further increase the local concentration of negatively charged lipid, which in turn nucleates membrane curvature and formation of caveolae structure by the way of electrostatic cavins-cavins or cavins-membrane interactions (reviewed in [41, 42]).

While caveolin-3-deficient mice exhibit robust muscular degeneration [43], the relative contribution of caveolae in protection against stretch-induced mechanical deformation is therefore difficult to judge. An attractive, albeit speculative, hypothesis is that caveolae are involved in both wound prevention and healing. Firstly, they can act as a membrane reserve that buffers the cell against local or global increases in in-plane membrane tension. Secondly, they can passively potentiate plasma membrane repair by releasing apparent membrane tension near the wound edge, a site of initial high membrane tension because of both high line tension and MCA-related tether forces. Finally, caveolae are also known to play central roles in dysferlin-mediated exocytosis (see Section 4.1.3.1) and the endocytic removal of bacterial pores ([44, 45]; see Section 4.2.1) and small mechanical lesions ([45]; see Section 4.1.3.2).
