Although proteolipid protein (PLP) and its DM20 isoform are the major membrane proteins of CNS myelin, their absence causes surprisingly few developmental defects. In comparison, missense mutations of the X-linked Plp gene cause severe dysmyelination. Previous studies have established roles for PLP/DM20 in the formation of the intraperiod line and in maintaining axonal integrity. 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 direct competition with wild-type oligodendrocytes that have a distinct advantage. PLP+ oligodendrocytes and PLP+ myelin sheaths make up the greater majority, 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 myelinate, remaining ensheathed but lacking a compact sheath, or show delayed myelination. These findings suggest that PLP/DM20 is also involved in the early stages of axon-oligodendrocyte interaction and wrapping of the axon.
Copyright 2001 Wiley-Liss, Inc.