Notch signaling without the APH-2/Nicastrin subunit of Gamma Secretase in C. elegans germline stem cells.

The final step in Notch signaling activation is the transmembrane cleavage of Notch receptor by γ secretase. Thus far, genetic and biochemical evidence indicate that four subunits are essential for γ secretase activity in vivo: presenilin (the catalytic core), APH-1, PEN-2, and APH-2/Nicastrin. Although some γ secretase activity has been detected in APH-2/Nicastrin-deficient mammalian cell lines, the lack of biological relevance for this activity has left the quaternary γ secretase model unchallenged. Here we provide the first example of in vivo Notch signal transduction without APH-2/Nicastrin. The surprising dispensability of APH-2/Nicastrin is observed in C. elegans germline stem cells (GSCs), and contrasts with its essential role in previously described C. elegans Notch signaling events. Depletion of GLP-1/Notch, presenilin, APH-1, or PEN-2 causes a striking loss of GSCs. In contrast, aph-2/Nicastrin mutants maintain GSCs, and exhibit robust and localized expression of the downstream Notch target sygl-1. Interestingly, APH-2/Nicastrin is present in GSCs and becomes essential under conditions of compromised Notch function. Further insight is provided by reconstituting the C. elegans γ secretase complex in yeast, where we find that APH-2/Nicastrin increases, but is not essential for γ secretase activity. Together, our results are most consistent with a revised model of γ secretase in which the APH-2/Nicastrin subunit has a modulatory, rather than obligatory role. We propose that a trimeric presenilin-APH-1-PEN-2 γ secretase complex can provide a low level of γ secretase activity, and that cellular context determines whether or not APH-2/Nicastrin is essential for effective Notch signal transduction.