Regulatory roles of presenilin-1 and nicastrin in neuronal differentiation during in vitro neurogenesis

Presenilin (PS) in association with nicastrin (NICA) forms a gamma-secretase complex that plays a crucial role in facilitating intramembranous processing of Notch, a signaling receptor that is essential for neuronal fate specification and differentiation. Loss of function studies have implicated a role for PS1 in regulating neuronal differentiation in association with the downregulation of Notch signaling during neurogenesis. By using a system for stable, as well as tetracycline-inducible expression of interfering RNAs (RNAi), we studied the functions of PS1 during neuronal differentiation in the murine pluripotent p19 embryonic carcinoma cell line. After retinoic acid ( RA) treatment and in the absence of doxycycline, neuronal progenitor cells in the p19 clone were found to extend their processes towards the neighboring colony to form network-like connections, as revealed by neuron-specific microtubule-associated protein 2 staining and laser scanning confocal microscopy. However, doxycycline-induced expression of PS1 small interfering RNA ( siRNA) in the p19 clone resulted in a severe defect in the formation of network-like connections. Expression of the NICA and Notch down-stream effector genes Hes1 and Hes5 was unaffected in p19 cells expressing doxycycline-induced PS1 siRNA. In contrast to PS1, constitutive inactivation of NICA by siRNA in p19 cells resulted in premature and partial differentiation without RA treatment. In these NICA siRNA-expressing p19 cells the expression of the Notch1 down-stream effector Hes1 gene was substantially reduced. After RA treatment the NICA siRNA clone failed to differentiate completely into networks of neurons. These results taken together provide direct evidence that PS1 and NICA may participate in neuronal differentiation during neurogenesis in vitro.