Active Expression of Retroelements in Neurons Differentiated from Adult Hippocampal Neural Stem Cells

In the mammalian brain, neurogenesis constitutively occurs in the subventricular zone (SVZ), the olfactory bulb, and the hippocampus throughout adulthood (Kuhn et al., 1996; Lois and Alvarez-Buylla, 1993; Gage, 2000; Pagano et al., 2000; Gritti et al., 2002), and adult hippocampal neurogenesis plays an important role in learning and memory (Deng et al., 2010). In the hippocampus, multipotent neural stem cells (NSCs) reside in the inner layer of the subgranular zone (SGZ) of the dentate gyrus (Gage, 2000; Suh et al., 2007). Undifferentiated NSCs express the high mobility group (HMG)-box transcription factor Sox2 (D'Amour and Gage, 2003; Suh et al., 2007). Sox2 is an SRY-related transcription factor encoding an HMG DNA-binding motif, and is expressed in embryonic stem (ES) cells and neural epithelial cells during development (Avilion et al., 2003; Ferri et al., 2004). Sox2 is essential for the multipotency and self-renewal capacity of NSCs and also functions in pluripotent ES cells (Bylund et al., 2003; Graham et al., 2003; Ferri et al., 2004). Sox2 is known to prevent neurogenesis during development and is thought to be critical for maintaining NSC populations in the neonatal brain (Bylund et al., 2003; Graham et al., 2003; Bani-Yaghoub et al., 2006). In the dentate gyrus of the hippocampus, Sox2 expression is found in the undifferentiated stem cell population with self-renewal capacity, and these Sox2-positive stem cells are exclusive with TUJ1-positive early stage of neurons (Fig. 1).