Mice lacking the expression of proteolipid protein (PLP)/DM20 in oligodendrocytes provide a genuine model for spastic paraplegia (SPG-2). Their axons are well myelinated but exhibit impaired axonal transport and progressive degeneration, which is difficult to attribute to the absence of a single myelin protein. We hypothesized that secondary molecular changes in PLP null myelin contribute to the loss of PLP/DM20-dependent neuroprotection and provide more insight into glia-axonal interactions in this disease model. By gel-based proteome analysis, we identified Ͼ160 proteins in purified myelin membranes, which allowed us to systematically monitor the CNS myelin proteome of adult PLP null mice, before the onset of disease. We identified three proteins of the septin family to be reduced in abundance, but the nicotinamide adenine dinucleotide (NAD ϩ )-dependent deacetylase sirtuin 2 (SIRT2) was virtually absent. SIRT2 is expressed throughout the oligodendrocyte lineage, and immunoelectron microscopy revealed its association with myelin. Loss of SIRT2 in PLP null was posttranscriptional, suggesting that PLP/DM20 is required for its transport into the myelin compartment. Because normal SIRT2 activity is controlled by the NAD ϩ /NADH ratio, its function may be coupled to the axo-glial metabolism and the long-term support of axons by oligodendrocytes.

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.

Using gene targeting, we inserted a high-affinity variant of the reverse tetracycline controlled transactivator (rtTA) into the genomic Sox10 locus. This rtTA transgene faithfully recapitulated Sox10 expression in the emerging neural crest, several of its derivatives, and in oligodendrocytes. It was furthermore able to induce expression of a tetracycline inducible transgenic reporter gene in a doxycycline-dependent manner. Induction was fast, with substantial reporter gene expression visible 6 h after the onset of doxycycline treatment. Shut-off, in contrast, exhibited delayed kinetics, which probably correlated with doxycycline clearance rates. This mouse provides a useful tool for generating tetracycline-controlled gene expression in neural crest and oligodendrocytes.

During myelin formation, vast amounts of specialized membrane proteins and lipids are trafficked toward the growing sheath in cell surface-directed transport vesicles. Soluble N-ethylmaleimide-sensitive factor (NSF) attachment proteins (SNAPs) are important components of molecular complexes required for membrane fusion. We have analyzed the expression profile and molecular interactions of SNAP-29 in the nervous system. In addition to its known enrichment in neuronal synapses, SNAP-29 is abundant in oligodendrocytes during myelination and in noncompact myelin of the peripheral nervous system. By yeast two-hybrid screen and coimmunoprecipitation, we found that the GTPases Rab3A, Rab24, and septin 4 bind to the N-terminal domain of SNAP-29. The interaction with Rab24 or septin 4 was GTP independent. In contrast, interaction between SNAP-29 and Rab3A was GTP dependent, and colocalization was extensive both in synapses and in myelinating glia. In HEK293 cells, cytoplasmic SNAP-29 pools were redistributed upon coexpression with Rab3A, and surface-directed trafficking of myelin proteolipid protein was enhanced by overexpression of SNAP-29 and Rab3A. Interestingly, the abundance of SNAP-29 in sciatic nerves was increased during remyelination and in a rat model of Charcot-Marie-Tooth disease, two pathological situations with increased myelin membrane biogenesis. We suggest that Rab3A may regulate SNAP-29-mediated membrane fusion during myelination.