SOX17 TRANSGENIC MICE REVEAL FUNCTIONAL SUPPRESSION OF BETA-CATENIN IN NEUROPROTECTION

In Multiple Sclerosis, damage to the myelin sheath and oligodendrocyte loss results in debilitating neurological symptoms. Oligodendrocytes are myelin-producing cells in the CNS white matter, whose membranes ensheath neuronal axons for rapid neurotransmission. Strategies for oligodendrocyte replacement will improve myelin repair in disease. The transcription factor Sox-17 has been shown to promote differentiation of oligodendrocyte progenitor cells (OPCs) into myelinating oligodendrocytes. Transgenic mice overexpressing Sox-17 in oligodendrocytes show enhanced regeneration of cells and protection from damage in white matter lesions. However, the signaling mechanism by which it promotes differentiation and white matter repair is unknown. Wnt/b-Catenin signaling inhibits differentiation of OPCs, which is supported by reports that demyelinating lesions show abnormally high b-Catenin levels. Sox-17 has been shown to inhibit the Wnt/b-Catenin pathway as well as increase white matter levels of Hedgehog mediator Gli2. Oligodendrocyte progenitor cells isolated from Sox-17-overexpressing transgenic mice show increased Hedgehog co-receptor clustering on cell membranes in response to agonist stimulation, suggesting Gli2 activation. We hypothesize that Sox-17 promotes OPC differentiation through Gli2 to antagonize b-Catenin signaling. To test this hypothesis, we performed mouse genotyping and analyzed experimental white matter lesions in Sox-17 transgenic mice. Lysophosphatidylcholine (Lyso), a detergent which disrupts myelin sheaths, was surgically injected into the white matter region of the brain to generate a demyelinated lesion and induce spontaneous repair. Using immunohistochemistry, we observed that Lyso injection results in increased cells expressing activated b-Catenin in unaltered wild type mice, whereas b-Catenin was activated by Lyso in significantly fewer cells in Sox-17 transgenic mice. To investigate possible antagonistic interactions between the transcription factors Gli2 and b-Catenin, we prepared plasmids and conducted transcription reporter assays in human oligodendroglioma cells. Using the b-Catenin responsive reporter Topflash, and a GLI-responsive reporter GliBSLuc, these transfections showed that, while b-Catenin activated Topflash, it had no effect on GliBSLuc. Gli2 however had differential effects. Gli2deltaN (active Gli2) functioned as an activator on GliBSLuc, but as a repressor on Topflash. In contrast, Gli2FL (Full length, inactivated Gli2) neither suppressed nor activated Gli or Topflash. This indicates that Gli2 activation is necessary for its repression of b-Catenin. We conclude that the presence of Sox-17 increases Gli2 activation, resulting in lower activation levels of b-Catenin signaling. This contributes to enhanced regeneration of oligodendrocytes, thus producing a protective effect in disease.