Electron cytochemical study of the muscle cell surface

Lack of anionic phospholipid calcium binding sites in Duchenne muscular dystrophy

We studied membrane ultrastructural localization of anionic phospholipids (AP) and sialic acid (SA) calcium binding sites in muscle biopsies from Duchenne muscular dystrophy (DMD) and 3 Becker's muscular dystrophy (BMD) patients using polymyxin B (PXB) and limulus polyphemus (LP) as cytochemical markers. We found that AP calcium binding sites are lacking at muscle cell surface in all DMD muscle tissues, in both intact and degenerating muscle fibers. In BMD, AP have an unusual distribution along plasma membrane. Sialic acid calcium binding sites have the same localization along plasma membrane and basal lamina in DMD, BMD, and control muscles. The absence or alterations of structures involved in calcium binding in DMD and BMD may alter membrane calcium permeability, leading to abnormal Ca2+ influx into cells causing muscle necrosis.

Ultrastructural localization of calcium binding sites on human muscle cell surface

Calcium (Ca2+) is mainly bound to anionic phospholipids and to sialic acid at the cell surface. We studied the ultrastructural localization of these Ca2+ binding sites in normal human muscle fibers, using Polymyxin B as a marker for anionic phospholipids and the lectin Limulus Polyphemus as a probe for sialic acid. We found that anionic phospholipids have a patchy distribution along the muscle sarcolemma, with a preferential localization at the I band level and at the junction between the I and A band. Sialic acid has an uniform distribution along the muscle plasma membrane and basal lamina. Our observations suggest that the plasma membrane, basal lamina, and transverse tubular system play an important role in providing the negative charge of the human muscle cell surface and that these structures may be involved in the binding of calcium.