Myelin proteolipid proteins promote the interaction of oligodendrocytes and axons

Although proteolipid protein (PLP) and its DM20 isoform are the major membr ane proteins of CNS myelin, their absence causes surprisingly few developme ntal defects. In comparison, missense mutations of the X-linked Plp gene ca use severe dysmyelination, Previous studies have established roles for PLP/ DM20 in the formation of the intraperiod line and in maintaining axonal int egrity. We now show that a normal number of oligodendrocytes are present in mice lacking PLP/DM20. However, in heterozygous females, which are natural chimeras for X-linked genes, oligodendrocytes lacking PLP/DM20 are in dire ct competition with wild-type oligodendrocytes that have a distinct advanta ge. PLP+ oligodendrocytes and PLP+ myelin sheaths make up the greater major ity, and this feature is generalised in the CNS throughout life. Moreover, in the absence of PLP/DM20, a proportion of small-diameter axons fails to m yelinate, remaining ensheathed but lacking a compact sheath, or show delaye d myelination. These findings suggest that PLP/DM20 is also involved in the early stages of axon-oligodendrocyte interaction and wrapping of the axon. (C) 2001 Wiley-Liss, Inc.