Myelin is a biologically active membrane receiving and processing signals from axons. Whereas, much is known about its structure and molecular composition, the intracellular signal transduction pathways, active during specific phases of myelinogenesis for regulating myelin formation remain poorly understood. Recent genetic loss-of-function studies have suggested a key role of extracelluar-signal-regulated-kinases-1 and -2 (ERK1/2), downstream mediators of mitogen-activated-protein-kinases (MAPKs), in promoting CNS and PNS myelination. In contrast, other studies, largely in vitro, have suggested that activation of ERK1/2 pathway can be detrimental for glial cell function and myelination. Given these conflicting reports, we investigated the effects of cell-autonomous activation of ERK1/2 in glial cells during developmental myelination in the intact CNS and PNS. Two lines of transgenic mice with sustained activation of ERK1/2 in oligodendrocyte progenitors (OPCs), oligodendrocytes, and Schwann cells were generated. Consistent with our loss-of-function studies, gain of ERK1/2 function in oligodendrocyte-lineage cells significantly increased myelin thickness, independent of oligodendrocyte differentiation or initiation of myelination. Additionally, increased activation of ERK1/2 in OPCs during early development resulted in transient hyper-proliferation and overproduction of OPCs, but generation of normal numbers of myelinating oligodendrocytes. Thus, these in vivo studies suggest a beneficial biphasic requirement of ERK1/2 during developmental myelination in the CNS, deployed first during early stages of the oligodendrocyte lineage for promoting OPC expansion and then redeployed later in myelinating oligodendrocytes for promoting myelin growth. Furthermore, Schwann cells with activated ERK1/2 hypermyelinate PNS axons, suggesting that ERK1/2 signaling is a conserved mechanism that promotes both CNS and PNS developmental myelination.

Wrapping of the myelin sheath around axons by oligodendrocytes is critical for the rapid conduction of electrical signals, required for the normal functioning of the central nervous system (CNS). Myelination is a multistep process where oligodendrocytes progress through a well-coordinated differentiation program regulated by multiple extracellular growth and differentiation signals. The intracellular-transduction of the extracellular signals that regulate myelination is poorly understood. Here we demonstrate a critical role for two important signaling molecules, extracelluar-signal-regulated-kinases-1 and -2 (ERK1/ERK2), downstream mediators of mitogen-activated protein kinases (MAPK), in the control of CNS myelin thickness. We generated and analyzed two lines of mice lacking both ERK1/ERK2 function specifically in oligodendrocyte-lineage cells. In the absence of ERK1/ERK2 signaling oligodendrocyte progenitor cells (OPC) proliferated and differentiated on schedule. Mutant oligodendrocytes also ensheathed axons normally and made a few wraps of compact myelin. However, the subsequent increase in myelination that correlated myelin thickness in proportion to the axon caliber failed to occur. Furthermore, although the numbers of differentiated oligodendrocytes in the adult mutants were unchanged, they showed an inability to upregulate the transcription of major myelin genes that normally occurs during active myelination. Similarly, in vitro ERK1/ERK2 deficient NG2+ oligodendrocytes differentiated normally but failed to form typical myelin-like membrane sheets. None of these effects were observed in single ERK1 or ERK2 mutants. These studies suggest that the predominant role of ERK1/ERK2 signaling in vivo is in promoting rapid myelin growth to increase its thickness, subsequent to oligodendrocyte differentiation and the initiation of myelination.

Motoneurons and oligodendrocytes in the embryonic spinal cord are produced from a restricted domain of the ventral ventricular zone, termed the pMN domain. The pMN domain is the site of expression of two basic helix-loop-helix transcription factors, Olig1 and Olig2, which are essential for motoneuron and oligodendrocyte development. Previous lineage-tracing experiments using Olig1-Cre and Olig2-GFP mice suggested that motoneurons and oligodendrocytes, but not astrocytes, are produced from the pMN domain. However, important questions remain, including the fate of neuroepithelial cells in the pMN domain, and specifically whether motoneurons and oligodendrocytes are the only types of cells produced in the pMN domain. We performed lineage-tracing experiments using a tamoxifen-inducible Cre-recombinase inserted into the Olig2 locus. We demonstrated that motoneurons and oligodendrocyte progenitors are derived from the Olig2+ progenitors in the pMN domain, and also found that a subset of astrocytes at the ventral surface of the spinal cord and ependymal cells at the ventricular surface are also produced from the pMN domain. These findings demonstrate that motoneurons and oligodendrocytes are not the only cell types originating from this domain.

Formation of the central nervous system (CNS) white matter is developmentally tightly regulated, but the molecules and mechanisms of myelination control in the postnatal CNS are poorly understood. Here, we show that myelin growth is controlled by Fibroblast Growth Factor (FGF) signaling, originally identified as a proliferative signal for oligodendrocyte precursor cells (OPC) in vitro. We created two lines of mice lacking both FGF-receptor 1 (Fgfr1) and Fgfr2 in oligodendrocyte lineage cells but found that in these mice OPC proliferation and differentiation were unaffected. Also axonal ensheathment and the initiation of myelination was on time. However, the rapid growth of CNS myelin, normally occurring in the second postnatal week, was strongly inhibited. Throughout adulthood, the myelin sheath remained disproportionately thin relative to the axon caliber. In adult mice, mutant oligodendrocytes were normal in number, whereas the transcription of major myelin genes was reduced. This FGF-receptor mediated stimulation of mature oligodendrocytes could also be modeled in vitro, demonstrating that enhanced expansion of oligodendroglial processes requires signaling by extracellular-signal regulated kinases-1 and -2 (Erk1/2), downstream mediaters of Mitogen-Activated Protein Kinase (MAPK). Also in vivo, Erk1/2-MAPK activity was reduced in the hypomyelinated CNS of Fgfr1/Fgfr2 mutant mice. These studies reveal a previously unrecognized function of FGF-receptor signaling in oligodendrocytes that contributes to the regulation of myelin sheath thickness, and which uncouples the initiation of ensheathment from the later phase of continued myelin growth.

Olig2 is a basic helix-loop-helix transcription factor essential for oligodendrocyte and motoneuron development in the spinal cord. Olig2-positive (Olig2+) cells in the ventricular zone of the ventral telencephalon have been shown to differentiate into GABAergic and cholinergic neurons. However, the fate of Olig2 lineage cells in the postnatal forebrain has not been fully described and Olig2 may regulate the development of both astrocytes and oligodendrocytes. Here, we examined the fate of embryonic Olig2+ progenitors using a tamoxifen-inducible Cre/loxP system. Using long-term lineage tracing, Olig2+ cells in the early fetal stage primarily differentiated into GABAergic neurons in the adult telencephalon, while those in later stages gave rise to macroglial cells, both astrocytes and oligodendrocytes. Olig2+ progenitors in the diencephalon developed into oligodendrocytes, as observed in the spinal cord, and a fraction developed into glutamatergic neurons. Olig2 lineage oligodendrocytes tended to form clusters, probably due to local proliferation at the site of terminal differentiation. In spite of the abundance of Olig2 lineage GABAergic neurons in the normal neocortex, GABAergic neurons seemed to develop at normal density in the Olig2 deficient mouse. Thus, Olig2 is dispensable for GABAergic neuron specification. In contrast, at the late fetal stage in the Olig2 deficient mouse, astrocyte development was retarded in the dorsal neocortex, but not in the basal forebrain. Olig2 functions, therefore, in gliogenesis in the dorsal pallium. Short-term lineage tracing experiments revealed that the majority of late Olig2+ cells were not direct descendants of early Olig2+ progenitors in the fetal forebrain. These observations indicate that embryonic Olig2+ progenitor cells change their differentiative properties during development, and also that Olig2 plays a role in astrocyte development in a region-specific manner.