The cytosolic loop of the γ-secretase component presenilin enhancer 2 protects zebrafish embryos from apoptosis

The γ-secretase complex, composed of presenilin, presenilin enhancer 2 (Pen-2), nicastrin, and Aph-1, catalyzes the final cleavage of amyloid precursor protein to generate the toxic amyloid β protein, the major component of plaques in the brains of Alzheimer disease patients. To understand the in vivo function of Pen-2, we used morphant technology available in zebrafish and transiently knocked down the expression of endogenous Pen-2 by injecting the morpholino (MO) against Pen-2. Two truncated Pen-2 proteins lacking either the cytosolic or the C-terminal domain were expressed in MO-injected embryos. This deletion analysis demonstrated that the Pen-2 cytosolic loop is essential for protecting developing embryos from caspase-dependent apoptosis caused by the reduction of Pen-2. Twelve amino acids in the C terminus of Pen-2 were dispensable and could not rescue the Pen-2 knockdown-induced apoptotic phenotype. Surprisingly, double knockdown of Pen-2 and nuclear factor κB component p65 abrogated the single Pen-2 MO-induced caspase activation, indicating that a previously reported pro-apoptotic role of NF-κB in some cell types could be manifested in a whole animal and that knockdown of Pen-2 may trigger pro-apoptotic activation of NF-κB.