Academic journal article Genetics

Presenilin-Based Genetic Screens in Drosophila Melanogaster Identify Novel Notch Pathway Modifiers

Academic journal article Genetics

Presenilin-Based Genetic Screens in Drosophila Melanogaster Identify Novel Notch Pathway Modifiers

Article excerpt

ABSTRACT

Presenilin is the enzymatic component of γ-secretase, a multisubunit intramembrane protease that processes several transmembrane receptors, such as the amyloid precursor protein (APP). Mutations in human Presenilins lead to altered APP cleavage and early-onset Alzheimer's disease. Presenilins also play an essential role in Notch receptor cleavage and signaling. The Notch pathway is a highly conserved signaling pathway that functions during the development of multicellular organisms, including vertebrates, Drosophila, and C. elegans. Recent studies have shown that Notch signaling is sensitive to perturbations in subcellular trafficking, although the specific mechanisms are largely unknown. To identify genes that regulate Notch pathway function, we have performed two genetic screens in Drosophila for modifiers of Presenilin-dependent Notch phenotypes. We describe here the cloning and identification of 19 modifiers, including nicastrin and several genes with previously undescribed involvement in Notch biology. The predicted functions of these newly identified genes are consistent with extracellular matrix and vesicular trafficking mechanisms in Presenilin and Notch pathway regulation and suggest a novel role for γ-tubulin in the pathway.

THE Presenilin genes encode eight-pass transmembrane proteins found in most metazoans, including mammals, Drosophila, and Caenorhabditis elegans (reviewed in SELKOE 2000; WOLFE and KOPAN 2004). In humans, mutations in the two Presenilin genes, PSl and PS2, account for the majority of familial early-onset Alzheimer's disease (reviewed in TANZI and BERTRAM 2001). Presenilin is the catalytic component of the "ysecretase complex that is responsible for the cleavage of the transmembrane protein, amyloid precursor protein (APP) (reviewed in DE STROOPER 2003). APP cleavage, first by â-secretase and subsequently by ~/-secretase, results primarily in the release of the 40-amino-acid amyloid â-peptide (Áâ40). Alzheimer's disease-associated mutations in PSl or PS2 subtly alter this cleavage pattern, causing increased production of a longer, more cytotoxic form of the amyloid â-peptide (Áâ42). Áâpeptides are the major component of amyloid plaques in the brains of Alzheimer's disease patients. Higher Áâ42 levels are thought to accelerate the aggregation of Áâ into toxic oligomers and the deposition of extracellular plaque material (reviewed in WOLFE and HAASS 2001; SELKOE 2004).

The -y-secretase complex is composed of at least three proteins in addition to Presenilin: nicastrin, aph-1, and pen-2 (YU et al. 2000; FRANCIS et al. 2002; GOUTTE et al. 2002). These four transmembrane proteins constitute the -y-secretase core complex, yet little is known about its regulation and activity. -y-secretase recognizes and cleaves a growing list of transmembrane proteins with very short extracellular domains generated by prior processing (STRUHL and ADACHI 2000; reviewed in DE STROOPER 2003; WOLFE and KOPAN 2004). A functional role for -y-secretase cleavage has not been demonstrated for most substrates. In such cases, ·ãsecretase may serve simply to eliminate transmembrane stubs of proteins after extracellular domain shedding (STRUHL and ADACHI 2000). However, in the case of the Notch family of receptors, -y-secretase plays an essential role in signaling. Genetic studies in C. elegans initially established that Presenilin is required for Notch pathway signaling (LEVITAN and GREENWALD 1995; Li and GREENWALD 1997), and this has now been confirmed in Drosophila, mouse, and human systems (reviewed in WOLFE and KOPAN 2004). Following ligand binding and subsequent cleavage of Notch by ADAM/TACE proteins, -y-secretase cleavage of Notch results in the release of the Notch intracellular domain (NICD). NICD translocates to the nucleus where it activates transcription of target genes in conjunction with the Suppressor of Hairless [Su(H)] and mastermind proteins (reviewed in BARON 2003; KADESCH 2004; WENG and ASTER 2004). …

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