Characterization of wild-type and Alzheimer\u27s disease mutant presenilins in Drosophila

Mutations in human Presenilin 1 and Presenilin 2 are a common cause of early-onset familial Alzheimer\u27s disease. Studies on C. elegans and murine Presenilin suggest that Presenilin may be involved in Notch signaling. To investigate this possibility, we isolated a Drosophila Presenilin homologue and determined its spatial and temporal distribution as well as its localization relative to the fly Notch protein. Our analysis revealed that Psn is widely expressed throughout Drosophila development and is coexpressed with Notch in many tissues, supporting a possible functional link between Presenilin and Notch. To further understand the role that Presenilin plays in the Notch signaling pathway, we analyzed Presenilin loss-of-function mutant phenotypes and found that removal of Drosophila Presenilin causes lethal Notch-like phenotypes including maternal neurogenic effects during embryogenesis, loss of lateral inhibition within proneural cell clusters, and absence of wing margin formation. We show that Presenilin is required for the normal proteolytic production of carboxy-terminal Notch fragments that are associated with receptor signaling. To further model Alzhermer\u27s disease in Drosophila, we took two strategies. We first overexpressed wild-type Drosophila Presenilin and the Alzheimer\u27s disease-linked mutant forms of Presenilins in a variety of tissues during different developmental stages using a UAS-GAL4 system. Overexpression of these proteins causes phenotypes resembling those of Psn amorphic mutants including low levels of apoptosis and weak neurogenic phenotypes, suggesting that overexpression of Presenilin exerts a dominant negative effect when expressed at high levels. The Psn-induced apoptotic effect is cell autonomous and can be almost completely inhibited by coexpressing different forms of constitutively activated Notch, suggesting that the increased apoptosis may be due to a developmental mechanism that eliminates improperly specified cell types when Notch signaling is impaired. Alzheimer\u27s disease-linked mutant Presenilins are less effective at inducing apoptosis than wild-type Presenilin, supporting the notion that these mutations represent partial loss-of-functions mutations. We also established a genetic assay for functionally characterizing mutant human Presenilin variants by substituting endogenous Drosophila Presenilin with wild-type or mutant forms of Presenilin from different species on a one-to-one basis. In this assay, mutant forms of Presenilin from either Drosophila or human are less active than wild-type Presenilin, and only partially rescue the lethality and Notch-like phenotypes caused by loss of Presenilin function. Our studies of Presenilin function using a molecular genetic approach in Drosophila have proven to be very valuable for understanding normal Presenilin function and the biological significance of Alzheimer\u27s disease-linked Presenilin mutations