Identification of a Novel Basic Helix-Loop-Helix-PAS Factor, NXF, Reveals a Sim2 Competitive, Positive Regulatory Role in Dendritic-Cytoskeleton Modulator Drebrin Gene Expression

Sim2, a basic helix-loop-helix (bHLH)-PAS transcriptional repressor, is thought to be involved in some symptoms of Down's syndrome. In the course of searching for hypothetical Sim2 relatives, we isolated another bHLH-PAS factor, NXF. NXF was a novel gene and was selectively expressed in neuronal tissues. While no striking homolog of NXF was found in vertebrates, a Caenorhabditis elegans putative transcription factor, C15C8.2, showed similarity in the bHLH-PAS domain. NXF had an activation domain as a transcription activator, and Arnt-type bHLH-PAS subfamily members were identified as the heterodimer partners of NXF. The NXF/Arnt heterodimer was capable of binding and activating a subset of Sim2/Arnt target DNA variants, and Sim2 could compete with the NXF activity on the elements. We showed that Drebrin had several such NXF/Arnt binding elements on the promoter, which could be direct or indirect cross talking points between NXF (activation) and Sim2 (repression) action. Drebrin has been reported to be engaged in dendritic-cytoskeleton modulation at synapses, and such a novel NXF signaling system on neural gene promoter may be a molecular target of the adverse effects of Sim2 in the mental retardation of Down's syndrome

Thy1-Positive Cells Have Bipotential Ability to Differentiate into Hepatocytes and Biliary Epithelial Cells in Galactosamine-Induced Rat Liver Regeneration

In galactosamine (GalN)-induced rat liver injury, hepatic stem/progenitor cells, small hepatocytes (SHs) and oval cells, transiently appear in the initial period of liver regeneration. To clarify the relationship between SHs and oval cells, CD44+ and Thy1+ cells were sorted from GalN-treated livers and used as candidates for SHs and oval cells, respectively. Some Thy1+ cells isolated 3 days after GalN-treatment (GalN-D3) formed CD44+ cell colonies, but those from GalN-D2 could form few. GeneChip (Affymetrix, Inc, Santa Clara, CA) analysis of the sorted cells and cultured Thy1+ cells suggested that hepatocytic differentiation progressed in the order Thy1+ (GalN-D3), Thy1+ cell colony (Thy1-C), and CD44+ (GalN-D4) cells. When Thy1+, Thy1-C, and CD44+ cells were transplanted into retrorsine/PH rat livers, they could proliferate to form hepatocytic foci. At 30 days after transplantation most cells forming the foci derived from CD44+ cells possessed C/EBPα+ nuclei, whereas only a few cells derived from Thy1-C showed this positivity. When Thy1+ (GalN-D3) cells were cultured between collagen gels in medium with hepatocyte growth factor+/dexamethasone−/dimethyl sulfoxide−, ducts/cysts consisting of biliary epithelial cells appeared, whereas with CD44+ and Thy1+ (GalN-D2) cells they did not. Taken together, these results indicate that the commitment of Thy1+ cells to differentiate into hepatocytes or biliary epithelial cells may occur between Day 2 and Day 3. Furthermore, some Thy1+ cells may differentiate into hepatocytes via CD44+ SHs