Neural stem cells lose telomerase activity upon differentiating into astrocytes

Serum-free mouse embryo (SFME) cells were established by D. Barnes et al., and are known to be a neural stem cell line, which differentiate into astrocytes upon treatment with TGF-beta. Therefore, SFME cells is thought to be a model well suited to analyze the differentiation mechanism of neural stem cells. Until now, we have investigated the regulation mechanisms of telomerase activity and telomere length in human cancer and normal cells. Telomerase is the enzyme responsible for the synthesis and maintenance of telomere repeats located at chromosomal ends and is normally expressed in embryonic and germline cells, but not in most normal cells. Here, using SFME cells, we attempted to analyze the regulation mechanism of telomerase activity in neural stem cells and to detect a change upon differentiation into astrocytes. When SFME cells were cultured in the presence of TGF-beta, cells showed an elongated morphology and decreased its growth to 50% of control culture. Cells also expressed the glial fibrillary acidic protein (GFAP), a marker for astrocytes, indicating that TGF-beta induced differentiation in SFME cells from neural stem cells into astrocytes. At the same time, TGF-beta also inhibited telomerase activity and repressed the expression of the mouse telomerase reverse transcriptase (mTERT), demonstrating that SFME cells was vested with a finite replicative life span upon treatment with TGF-beta. To understand the mechanisms regulating mTERT levels during differentiation into astrocytes, we have estimated the expression level of c-myc, which is known to be a key molecule in activating the TERT promoter. As a result, TGF-beta-treated SFME cells were shown to repress the expression of c-myc. Furthermore, promoter analysis, using the 5'-region of the mTERT gene, which possess two E-box elements bound to c-Myc/Max, demonstrated that mTERT promoter activity greatly decreased in TGF-beta-treated SFME cells as compared to non-treated SFME cells. These suggest that c-myc might play a critical role in the expression of mTERT, and that down-regulation of c-myc dependent upon the astrocytic differentiation in SFME cells might cause the repression of mTERT in TGF-beta-treated SFME cells.