Membrane interactions are altered in myelin isolated from central and peripheral nervous system tissues

Isolated myelin has been used for determinations of membrane surface charge density and topographical mapping of components in the membrane. To determine how similar such myelin is to myelin of intact tissue, we have used x-ray diffraction to compare their intermembrane interactions. The interactions were monitored by measuring the myelin period in samples treated with distilled water, buffered saline at pH 4-9 and ionic strength 0.06-0.18, and saline containing HgCl2 or triethyl tin sulfate. Myelin was isolated from whole brains and sciatic nerves of mice by conventional methods involving sucrose gradient centrifugation and osmotic shock. Consistent with previous findings, electron microscopy showed that the multilamellar morphology, staining, and repeat periods of isolated myelin were essentially like those of intact myelin; however, the membrane stacks were less extensive than those in whole tissue. X-ray diffraction revealed that isolated CNS myelin was like intact myelin in showing reversible compaction in acidic media and in distilled water. However, unlike the myelin in whole tissue, isolated CNS myelin did not swell in hypotonic or alkaline media, or in the presence of HgCl2-saline or triethyl tin. The altered membrane interactions could result from an increase in adhesiveness of the apposed membrane surfaces. Reorganization of proteolipid protein and/or a reduction of surface charge could account for the change in surface properties of isolated CNS myelin. Isolated PNS myelin, like the membranes in whole tissue, showed both compaction and swelling; however, the membrane pairs were disordered in the swollen structure. This irregular membrane swelling could result from charge variation in the extracellular surfaces.