Conversion of the cellular prion protein (PrPC) into the pathogenic isoform (PrPSc) is the fundamental event underlying transmission and pathogenesis of prion diseases. These methods take advantage of the fact that the proliferative neural progenitor cells (NPCs) are more sensitive than differentiated neurons to these insults, which disrupt cell cycle progression. The viral tegument protein VP16 transactivates the expression of the five IE genes. (c,d) Immunoblot for LAMP-2A in liver and kidney homogenates (c) or for L-2A and LAMP-1 (L-1) in liver homogenates (d) from similar mouse groups as in b switched to doxycycline-free food for 4 weeks. Regulated GDNF expression was highly sensitive to dietary doxycycline (DOX), displaying undetectable striatal GDNF levels at serum DOX levels below those required for antimicrobial activity. We observed stable inducible expression for over 1 month, with uniform expression in the cell population and between clones derived from the same integration site. Although low levels of background expression may be tolerable in other gene-therapy strategies for neurological disorders, given the relatively long in vivo half-life of GDNF, any in vivo leakiness will lead to unpredictable GDNF accumulation.
Gene silencing based on short interfering RNAs (siRNA) has been demonstrated in several organisms, including in cultured mammalian cells8, and works by targeting complementary messenger RNA (mRNA) thereby activating the RISC complex causing sequence-specific mRNA degradation.