Transcriptional regulation of TRKC by SOX2 in human embryonic stem cells

Human embryonic stem (hES) cells have the dual ability to self-renew and differentiate into specialized cell types. However, in order to realize the full potential of these cells it is important to understand how the genes responsible for their unique characteristics are regulated. In this study we examine the regulation of the tropomyosin-related kinase (TRK) genes which encode for receptors important in hES cell survival and self-renewal. Although the TRK genes have been studied in many neuronal cell types, the regulation of these genes in hES cells is unclear. Our study demonstrates a novel regulatory relationship between the TRKC gene and the transcription factor SOX2. Our results found that hES cells highly express full-length and truncated forms of the TRKC gene. However, examination of the related TRKB gene showed a lower overall expression of both full-length and truncated forms. Through RNA interference, we knocked down expression levels of SOX2 in hES cells and examined the expression of TRKC, as well as TRKB. Upon loss of SOX2 we found that TRKC mRNA levels were significantly downregulated but TRKB levels remained unchanged, demonstrating an important regulatory dependence on SOX2 by TRKC. We also found that TRKC protein levels were also decreased after SOX2 knock down. Further analysis found the regulatory region of TRKC to be highly conserved among many mammals with potential SOX binding motifs. We confirmed a specific binding motif as a site that SOX2 utilizes to directly interact with the TRKC regulatory region. In addition, we found that SOX2 drives expression of the TRKC gene by activating a luciferase reporter construct containing the TRKC regulatory region and the SOX binding motif.