Two proteins-the recently described aph-1 gene product (see Goutte, et al. ) as well as a novel protein-likely play a role in the production of amyloid-beta (Aβ) via the presenilin/γ-secretase pathway, according to a study in the July of issue of Developmental Cell.
Sometimes overlooked in the brouhaha over whether presenilin is the γ-secretase is the fact that there are other, anonymous proteins hovering around the pathway that generates Aβ from the amyloid precursor protein (AβPP). There is clear evidence that presenilin and γ-secretase cofractionate with other proteins, and only one of these-nicastrin or APH-2-has been positively identified as being critical for AβPP processing. However, recent work in the Notch signaling cascade, which is critical throughout ontogeny and which seems dependent on the same γ-secretase, has pointed out another accessory, termed APH-1, as necessary for γ-secretase activity.
In the current study, Daniel Curtis, Ross Francis, and colleagues at Exelixis, Inc., in South San Francisco and at Pharmacia’s Discovery Research in Kalamazoo, Michigan, screened the genome of mutagenized C. elegans worms and found that aph-1 and a novel gene termed presenilin enhancer 2 (pen-2) interact with presenilin. The aph-1 gene encodes a protein predicted to consist of 308 amino acids, with seven transmembrane segments, whereas pen-2 is predicted to encode a protein of 101 amino acids, with two transmembrane segments.
Both of these C. elegans genes were found to have human homologues, two of which were mapped to loci implicated in Alzheimer’s disease on chromosomes 1 and 19. When they looked in human tissues, the researchers found that the human aph-1 and pen-2 homologues were broadly expressed, including in brain. Localization experiments in C. elegans, successful for pen-2 only, found the gene product in intracellular membrane compartments.
Further experiments in C. elegans mutants showed that the aph-1 and pen-2 genes are not only critical to proper Notch pathway functioning, but they specifically interact with the presenilin and aph-2/nicastrin genes. In the Notch pathway, aph-1 and pen-2 were shown to act at, or upstream of, the S3 cleavage step that appears to be the γ-secretase step.
Switching to cultured Drosophila cells, the researchers showed that inactivation of either aph-1, pen-1, or aph-2/nicastrin reduces γ-secretase cleavage of AβPP, reduces processing of presenilin, and lowers the production of Aβ-40 and -42.
“If APH-1 and PEN-2 proteins interact directly with each other and with presenilin, these interactions could be transitory during early steps in γ-secretase complex assembly or may persist in the mature enzyme complex,” note the authors.—Hakon Heimer