Myelin palingenesis. 1. Electron microscopical localization of myelin/oligodendrocyte proteins in multilamellar...

The issue of the capacity of mature oligodendrocytes to remyelinate naked axons has not been totally resolved. The impression is that for this to happen, oligodendrocytes have to undergo cell division. We are interested in providing an answer to the question: can oligodendrocytes myelinate more than once? To address this question, we are using a model system consisting of pure cultures of postmyelination oligodendrocytes. We have previously shown that these cells have plasticity and are able to regenerate, but for this to occur they need a signal. In vitro, this signal is provided by interaction with a positively charged substratum and a serum factor(s). Over time in culture, without cell division, oligodendrocytes assemble multilamellar structures that ultrastructurally resemble myelin. We have named this process myelin palingenesis--rebirth--to distinguish it from remyelination. In order to ascertain the significance of the regenerative process observed in vitro and its predictive value as to the capabilities of oligodendrocytes to ensheathe axons, we undertook an ultrastructural and immunocytochemical characterization of the multilamellar membranes to assess whether they contain all the myelin-characteristic proteins, i.e., myelin basic protein, proteolipid protein, 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin-associated glycoprotein. We have used antibodies against these proteins as immunocytochemical probes in conjunction with the immunogold method at the electron microscopic level. Oligodendrocyte cultures were processed for electron microscopy. Blocks were serially sectioned parallel to the culture plate at 0.09-micron spacings. Staining of cells was done prior to embedding. Analysis of these micrographs brought to light the existence of membrane-membrane interaction between multilamellar structures; this is reminiscent of the interactions observed in situ between myelinated axons. We have found that the membranous structures possess all the myelin-characteristic proteins. Furthermore, based on the accessibility of these proteins to the antibodies, e.g., whether or not permeabilization was required, we can surmise that these proteins are incorporated into the membranous structures with the same orientation as in myelin. These results, in conjunction with our earlier work, show that mature oligodendrocytes stripped of their myelin are able to regenerate and reassemble multilamellar membranes that have all the myelin proteins. It remains to be proven that when provided with axons, these oligodendrocytes will generate typical myelin.