Sustained ErbB Activation Causes Demyelination and Hypomyelination by Driving Necroptosis of Mature Oligodendrocytes and Apoptosis of Oligodendrocyte Precursor Cells.

Oligodendrocytes are vulnerable to genetic and environmental insults and its injury leads to demyelinating diseases. The roles of ErbB receptors in maintaining the CNS myelin integrity are largely unknown. Here, we overactivate ErbB receptors that mediate signaling of either neuregulin (NRG) or epidermal growth factor (EGF) family growth factors and found their synergistic activation caused deleterious outcomes in white matter. Sustained ErbB activation induced by the tetracycline-dependent mouse tool Plp-tTA resulted in demyelination, axonal degeneration, oligodendrocyte precursor cell (OPC) proliferation, astrogliosis, and microgliosis in white matter. Moreover, there was hypermyelination before these inflammatory pathologic events. In contrast, sustained ErbB activation induced by another tetracycline-dependent mouse tool Sox10+/rtTA caused hypomyelination in the corpus callosum and optic nerve, which appeared to be a developmental deficit and did not associate with OPC regeneration, astrogliosis, or microgliosis. By tracing the differentiation states of cells expressing tetracycline-controlled transcriptional activator (tTA)/reverse tTA (rtTA)-dependent transgene or pulse-labeled reporter proteins in vitro and in vivo, we found that Plp-tTA targeted mainly mature oligodendrocytes (MOs), whereas Sox10+/rtTA targeted OPCs and newly-formed oligodendrocytes (NFOs). The distinct phenotypes of mice with ErbB overactivation induced by Plp-tTA and Sox10+/rtTA consolidated their nonoverlapping targeting preferences in the oligodendrocyte lineage, and enabled us to demonstrate that ErbB overactivation in MOs induced necroptosis that caused inflammatory demyelination, whereas in OPCs induced apoptosis that caused noninflammatory hypomyelination. Early interference with aberrant ErbB activation ceased oligodendrocyte deaths and restored myelin development in both mice. This study suggests that aberrant ErbB activation is an upstream pathogenetic mechanism of demyelinating diseases, providing a potential therapeutic target.SIGNIFICANCE STATEMENT Primary oligodendropathy is one of the etiologic mechanisms for multiple sclerosis, and oligodendrocyte necroptosis is a pathologic hallmark in the disease. Moreover, the demyelinating disease is now a broad concept that embraces schizophrenia, in which white matter lesions are an emerging feature. ErbB overactivation has been implicated in schizophrenia by genetic analysis and postmortem studies. This study suggests the etiologic implications of ErbB overactivation in myelin pathogenesis and elucidates the pathogenetic mechanisms.

Interference of commissural connections through the genu of the corpus callosum specifically impairs sensorimotor gating.

White matter abnormalities in schizophrenic patients are characterized as regional tract-specific. Myelin loss at the genu of the corpus callosum (GCC) is one of the most consistent findings in schizophrenic patients across the different populations. We characterized the axons that pass through the GCC by stereotactically injecting an anterograde axonal tracing viral vector into the forceps minor of the corpus callosum in one hemisphere, and identified the homotopic brain structures that have commissural connections in the two hemispheres of the prefrontal cortex, including the anterior cingulate area, the prelimbic area, the secondary motor area, and the dorsal part of the agranular insular area, along with commissural connections with the primary motor area, caudoputamen, and claustrum. To investigate whether dysmyelination in these commissural connections is critical for the development of schizophrenia symptoms, we generated a mouse model with focal demyelination at the GCC by stereotactically injecting demyelinating agent lysolecithin into this site, and tested these mice in a battery of behavioral tasks that are used to model the schizophrenia-like symptom domains. We found that demyelination at the GCC influenced neither the social interest or mood state, nor the locomotive activity or motor coordination. Nevertheless, it specifically reduced the prepulse inhibition of acoustic startle that is a well-known measure of sensorimotor gating. This study advances our understanding of the pathophysiological contributions of the GCC-specific white matter lesion to the related disease, and demonstrates an indispensable role of interhemispheric communication between the frontal cortices for the top-down regulation of the sensorimotor gating.

Differential Inhibition of Sox10 Functions by Notch-Hes Pathway.

In the developing central nervous system, the terminal differentiation of oligodendrocytes (OLs) is regulated by both extrinsic and intrinsic factors. Recent studies have suggested that the Notch-Hes signaling pathway influences the maturation of oligodendrocytes in culture and during development. However, the specific Notch receptors and their downstream effectors Hes genes that are involved in oligodendrocyte maturation have not been investigated systematically. In this study, we showed that Notch1 and Notch3 are expressed in oligodendrocyte precursor cells (OPCs) during gliogenesis, and Hes5 is the major Notch downstream transcription factor that is transiently expressed in OPCs. Overexpression of Notch intracellular domain (NICD) and Hes5 proteins in embryonic chicken spinal cord suppressed both the endogenous and Sox10-induced Mbp gene expression. Unexpectedly, overexpression of NICD/Hes5 did not inhibit Sox10 induction of Olig2 expression and Myrf induced Mbp expression, suggesting the differential inhibitory effects of NICD/Hes5 signaling on Sox10 activation of myelin-related genes and early progenitor genes.

Stage-dependent regulation of oligodendrocyte development and enhancement of myelin repair by dominant negative Master-mind 1 protein.

Notch signaling has been implicated in the inhibition of oligodendrocyte differentiation and myelin gene expression during early development. However, inactivation of a particular Notch or Hes gene only produces a mild phenotype in oligodendrocyte development possibly due to the functional redundancies among closely related family members. To uncover the full role of Notch signaling in myelin development and regeneration, we generated the Sox10rtTA/+ ; TetO-dnMAML1 double transgenic mice in which expression of dominant negative Master-mind 1 (dnMAML1) gene can be selectively induced in oligodendrocyte precursor cells (OPCs) for complete blockade of Notch signaling. It is found that dnMAML1 expression leads to robust precocious OL differentiation and premature axonal myelination in the spinal cord, possibly by upregulating Nkx2.2 and downregulating Pdgfra expression. Unexpectedly, at late embryonic stages, dnMAML1 expression dramatically increased the number of OPCs, indicating a stage-dependent effect of Notch signaling on OPC proliferation. In addition, dnMAML1 also significantly enhances axonal remyelination following chemical-induced demyelination, providing a promising therapeutic target for lesion repair in demyelinating disease.

MicroRNA-20a is essential for normal embryogenesis by targeting vsx1 mRNA in fish.

MicroRNAs are major post-transcriptional regulators of gene expression and have essential roles in diverse developmental processes. In vertebrates, some regulatory genes play different roles at different developmental stages. These genes are initially transcribed in a wide embryonic region but restricted within distinct cell types at subsequent stages during development. Therefore, post-transcriptional regulation is required for the transition from one developmental stage to the next and the establishment of different cell identities. However, the regulation of many multiple functional genes at post-transcription level during development remains unknown. Here we show that miR-20a can target the mRNA of vsx1, a multiple functional gene, at the 3'-UTR and inhibit protein expression in both goldfish and zebrafish. The expression of miR-20a is initiated ubiquitously at late gastrula stage and exhibits a tissue-specific pattern in the developing retina. Inhibition of vsx1 3'-UTR mediated protein expression occurs when and where miR-20a is expressed. Decoying miR-20a resulted in severely impaired head, eye and trunk formation in association with excessive generation of vsx1 marked neurons in the spinal cord and defects of somites in the mesoderm region. These results demonstrate that miR-20a is essential for normal embryogenesis by restricting Vsx1 expression in goldfish and zebrafish, and that post-transcriptional regulation is an essential mechanism for Vsx1 playing different roles in diverse developmental processes.

Sequence divergence in the 3'-untranslated region has an effect on the subfunctionalization of duplicate genes.

Recent studies demonstrated that sequence divergence in both transcriptional regulatory region and coding region contributes to the subfunctionalization of duplicate gene. However, whether sequence divergence in the 3'-untranslated region (3'-UTR) has an impact on the subfunctionalization of duplicate genes remains unclear. Here, we identified two diverging duplicate vsx1 (visual system homeobox-1) loci in goldfish, named vsx1A1 and vsx1A2. Phylogenetic analysis suggests that vsx1A1 and vsx1A2 may arise from a duplication of vsx1 after the separation of goldfish and zebrafish. Sequence comparison revealed that divergence in both transcriptional and translational regulatory regions is higher than divergence in the introns. vsx1A2 expresses during blastula and gastrula stages and in adult retina but silences from segmentation stage to hatching stage, vsx1A1 starts expression from segmentation onward. Comparing to that zebrafish vsx1 expresses in all the developmental stages and in the adult retina, it appears that goldfish vsx1A1 and vsx1A2 are under going to share the functions of ancestral vsx1. The different but overlapping temporal expression patterns of vsx1A1 and vsx1A2 suggest that sequence divergence in the promoter region of duplicate vsx1 is not sufficient for partitioning the functions of ancestral vsx1. By comparing vsx1A1 and vsx1A2 3'-UTR-linked green fluorescent protein gene expression patterns, we demonstrated that the 3'-UTR of vsx1A1 remains but the 3'-UTR of vsx1A2 has lost the capability of mediating bipolar cell specific expression during retina development. These results indicate that sequence divergence in the 3'-UTRs has a clear effect on subfunctionalization of the duplicate genes.