Image of myelin basic protein expression in the cerebellum of the adult mouse from the Allen Brain Atlas.
Do babies mature to adulthood in one long process or in several independent steps? Dugas et al. ask a similar question about the maturation of oligodendrocytes, which myelinate axons in the central nervous system. They report two distinct waves of gene expression in oligodendrocyte precursor cells (OPCs) undergoing differentiation in a recent article in the Journal of Neuroscience.
The authors cultured OPCs from the perinatal rat brain in media supplemented with growth factors. To encourage differentiation, they removed growth factors from the media and added thyroid hormone. The authors isolated RNA for microarrays from cultured OPCs immediately before and at various times after switching to differentiation media.
Of the over 26,000 genes on the microarray chip, only 824 were more than four-fold different in oligodendrocytes relative to OPCs. The differentiation process lasted approximately seven days. Whereas 244 genes changed by more than 150% between days five and seven in culture, only 63 genes changed by more than 150% between days seven and nine.
Mature oligodendrocytes isolated from ten-day-old rats had similar gene expression profiles as oligodendrocytes differentiated for nine days in vitro. The authors found only seven oligodendrocyte-specific genes that were induced in vivo and not in vitro.
The authors consulted the Allen Brain Atlas, the Brain Gene Expression Map and the Gene Expression Nervous System Atlas to localize the expression of genes most enriched in oligodendrocytes. Approximately 80% localized to white matter and nearly 50% showed elevated expression in white matter relative to the rest of the brain.
Genes involved in myelination were induced either at the onset of differentiation or following a delay of at least 48 hours. Many genes encoding transcription factors and cell cycle regulators showed expression patterns similar to the myelination genes, and many genes showed the opposite pattern, with reduced gene expression immediately following or 48 hours after differentiation began. Therefore, the authors concluded that oligodendrocyte differentiation occurs in two stages. To test the independence of these stages, they disrupted genes regulated early or late in oligodendrocyte differentiation. SOX10 is a transcription factor that regulates the earliest stages of oligodendrocyte differentiation. siRNA-mediated knock down of SOX10 expression blocked expression of myelin basic protein, which is expressed early in differentiation, but did not affect the expression of myelin oligodendrocyte glycoprotein, which is induced late in differentiation. In contrast, RNAi-mediated knock down of ZFP536, a transcription factor upregulated late in oligodendrocyte differentiation, reduced myelin oligodendrocyte glycoprotein expression, but did not affect expression of myelin basic protein. These data suggest that the stages of OPC differentiation into mature oligodendrocytes are discrete.
Several genes induced in oligodendrocyte differentiation localized to genetic regions implicated in demyelinating diseases. Selenoprotein P, plasma, 1 (SEPP1), which regulates selenium levels, was elevated 20-fold in oligodendrocytes relative to OPCs and localized to a genetic region implicated in multiple sclerosis. Therefore, the authors believe that mutations in SEPP1 might confer susceptibility to multiple sclerosis.