Annexin-A6 in Membrane Repair of Human Skeletal Muscle Cell: A Role in the Cap Subdomain

Abstract

Defects in membrane repair contribute to the development of some muscular dystrophies, highlighting the importance to decipher the membrane repair mechanisms in human skeletal muscle. In murine myofibers, the formation of a cap subdomain composed notably by annexins (Anx) is critical for membrane repair. We applied membrane damage by laser ablation to human skeletal muscle cells and assessed the behavior of annexin-A6 (AnxA6) tagged with GFP by correlative light and electron microscopy (CLEM). We show that AnxA6 was recruited to the site of membrane injury within a few seconds after membrane injury. In addition, we show that the deficiency in AnxA6 compromises human sarcolemma repair, demonstrating the crucial role played by AnxA6 in this process. An AnxA6-containing cap-subdomain was formed in damaged human myotubes in about one minute. Through transmission electron microscopy (TEM), we observed that extension of the sarcolemma occurred during membrane resealing, which participated in forming a dense lipid structure in order to plug the hole. By properties of membrane folding and curvature, AnxA6 helped in the formation of this tight structure. The compaction of intracellular membranes—which are used for membrane resealing and engulfed in extensions of the sarcolemma—may also facilitate elimination of the excess of lipid and protein material once cell membrane has been repaired. These data reinforce the role played by AnxA6 and the cap subdomain in membrane repair of skeletal muscle cells.

Annexins and Membrane Repair Dysfunctions in Muscular Dystrophies

“…Most observations of damaged myofibers, notably ours by transmission electron microscopy [ 24 , 25 , 26 ], have shown accumulation of lipid material at the site of membrane disruption, suggesting a repair process according to the “lipid patch” model proposed by P. McNeil in the late 1990s [ 27 , 28 ]. In this model, membrane resealing is based on the recruitment and accumulation of cytosolic vesicles.…”

“…In this model, membrane resealing is based on the recruitment and accumulation of cytosolic vesicles. This process may be combined with an elongation of the sarcolemma [ 26 ], acting in concert to seal the rupture. The whole thing could be driven by a protein scaffold consisting of at least two parts, identified as the cap and shoulder subdomains [ 26 , 29 ].…”

Plasma membrane integrity: implications for health and disease

“…Plug formation has been observed in response to small and large breaches, where it is proposed to limit exchange of material between the cytoplasm and the extracellular space while other forms of repair proceed. A role for plugging is supported by the accumulation of tightly opposed, membranous compartments at wound sites in essentially all cell types [ 109 , 120 , 152 , 161 ]. Both annexins (ANXA) and dysferlin are promising candidates to promote plugging as they are calcium-dependent, membrane-binding proteins recruited to the wound site with the potential to crosslink vesicles to each other or the plasma membrane [ 110 , 111 , 120 , 162 , 163 ].…”

“…Sarcolemma resealing is also facilitated by ESCRT-III activity and annexin recruitment, whereby annexins form an actin-dependent structure termed a repair cap [ 167 , 374 , 375 ]. Based on electron micrographs of this structure [ 161 ], the cap is likely equivalent to the complex network of membrane protrusions seen at wounds in other cell types [ 109 , 153 ]. The accumulation of MG53, dysferlin, EHD1, and BIN1 adjacent to the repair cap suggests simultaneous vesicle fusion events and membrane remodeling to relieve tension during annexin-mediated closure [ 167 , 376 ].…”

Special Issue “Recent Developments in Annexin Biology”

“…Expanding on the lipid and membrane interaction role of AnxA2, Bittel et al [4] show the importance of the lipid-, protein-and calcium-binding ability of AnxA2 in sensing injury to the plasma membrane, and coordinating a cellular repair response by facilitating injury-triggered vesicle fusion. The role of annexins in membrane repair is also examined by Croissant et al [5], who make use of correlative light and electron microscopy approaches. Through these studies they identify that plasma membrane injury causes an accumulation of AnxA6 at the site of injury, where it facilitates formation of a plug derived from the membranes at the injury site to help close the hole in the membrane.…”