The bHLH transcription factor Olig1 promotes oligodendrocyte maturation and is required for myelin repair. In this report, we characterize an Olig1-regulated G-protein coupled receptor GPR17 whose function is to oppose the action of Olig1. GPR17 is restricted to oligodendrocyte lineage cells but downregulated during the peak period of myelination and in adulthood. Transgenic mice with sustained GPR17 expression in oligodendrocytes exhibit stereotypic features of myelinating disorders in the CNS. GPR17 overexpression inhibits oligodendrocyte differentiation and maturation both in vivo and in vitro. Conversely, GPR17 knockout mice display early onset of oligodendrocyte myelination. The opposing action of GPR17 on oligodendrocyte maturation reflects, at least partially, upregulation and nuclear translocation of the potent oligodendrocyte differentiation inhibitors ID2/4. Collectively, these findings suggest that GPR17 orchestrates the transition between immature and myelinating oligodendrocytes via an ID protein-mediated negative regulation, and may serve as a potential therapeutic target for CNS myelin repair.

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This paper presents a circular microfluidic compartmentalized co-culture platform that can be used for central nervous system (CNS) axon myelination research. The microfluidic platform is composed of a soma compartment and an axon/glia compartment connected through arrays of axon-guiding microchannels. Myelin-producing glia, oligodendrocytes (OLs), placed in the axon/glia compartment, interact with only axons but not with neuronal somata confined to the soma compartment, reminiscent to in vivo situation where many axon fibres are myelinated by OLs at distance away from neuronal cell bodies. Primary forebrain neurons from embryonic day 16-18 rats were cultured inside the soma compartment for two weeks to allow them to mature and form extensive axon networks. OL progenitors, isolated from postnatal day 1-2 rat brains, were then added to the axon/glia compartment and co-cultured with neurons for an additional two weeks. The microdevice showed fluidic isolation between the two compartments and successfully isolated neuronal cell bodies and dendrites from axons growing through the arrays of axon-guiding microchannels into the axon/glia compartment. The circular co-culture device developed here showed excellent cell loading characteristics where significant numbers of cells were positioned near the axon-guiding microchannels. This significantly increased the probability of axons crossing these microchannels as demonstrated by the more than 51% of the area of the axon/glia compartment covered with axons two weeks after cell seeding. OL progenitors co-cultured with axons inside the axon/glia compartment successfully differentiated into mature OLs. These results indicate that this device can be used as an excellent in vitro co-culture platform for studying localized axon-glia interaction and signalling.

Reactive microglia and astrocytes are present in lesions of white matter disorders, such as periventricular leukomalacia and multiple sclerosis. However, it is not clear whether they are actively involved in the pathogenesis of these disorders. Previous studies demonstrated that microglia, but not astrocytes, are required for lipopolysaccharide (LPS)-induced selective killing of developing oligodendrocytes (preOLs) and that the toxicity is mediated by microglia-derived peroxynitrite. Here we report that, when astrocytes are present, the LPS-induced, microglia-dependent toxicity to preOLs is no longer mediated by peroxynitrite but instead by a mechanism dependent on tumor necrosis factor-␣ (TNF␣) signaling. Blocking peroxynitrite formation with nitric oxide synthase (NOS) inhibitors or a decomposition catalyst did not prevent LPS-induced loss of preOLs in mixed glial cultures. PreOLs were highly vulnerable to peroxynitrite; however, the presence of astrocytes prevented the toxicity. Whereas LPS failed to kill preOLs in cocultures of microglia and preOLs deficient in inducible NOS (iNOS) or gp91 phox , the catalytic subunit of the superoxide-generating NADPH oxidase, LPS caused a similar degree of preOL death in mixed glial cultures of wild-type, iNOS Ϫ/Ϫ , and gp91 phox؊/؊ mice. TNF␣ neutralizing antibody inhibited LPS toxicity, and addition of TNF␣ induced selective preOL injury in mixed glial cultures. Furthermore, disrupting the genes encoding TNF␣ or its receptors TNFR1/2 completely abolished the deleterious effect of LPS. Our results reveal that TNF␣ signaling, rather than peroxynitrite, is essential in LPS-triggered preOL death in an environment containing all major glial cell types and underscore the importance of intercellular communication in determining the mechanism underlying inflammatory preOL death.

Deletions in the DAP12 gene in humans result in Nasu-Hakola disease, characterized by a combination of bone fractures and psychotic symptoms similar to schizophrenia, rapidly progressing to presenile dementia. However, it is not known why these disorders develop upon deficiency in DAP12, an immunoreceptor signal activator protein initially identified in the immune system. Here we show that DAP12-deficient (DAP12–/–) mice develop an increased bone mass (osteopetrosis) and a reduction of myelin (hypomyelinosis) accentuated in the thalamus. In vitro osteoclast induction from DAP12–/– bone marrow cells yielded immature cells with attenuated bone resorption activity. Moreover, immature oligodendrocytes were arrested in the vicinity of the thalamus, suggesting that the primary defects in DAP12–/– mice are the developmental arrest of osteoclasts and oligodendrocytes. In addition, the mutant mice also showed synaptic degeneration, impaired prepulse inhibition, which is commonly observed in several neuropsychiatric diseases in humans including schizophrenia, and aberrant electrophysiological profiles in the thalami. These results provide a molecular basis for a unique combination of skeletal and psychotic characteristics of Nasu-Hakola disease as well as for schizophrenia and presenile dementia