Using genetic cross-breeding, Rozmahel et al. presented a novel finding that the presenilin 1 (PS1) developmental activity could be modified by genes located on the distal arm of chromosome 1. They further showed that one of the modifiers could be nicastrin, as the polymorphisms between 129 and C57BL/6J strains result in two missense substitutions in the nicastrin protein.
It would be very interesting to see how the two missense substitutions modify the functions of nicastrin and presenilins. Since the PS1 null phenotype does not seem to be affected by genetic background (our unpublished observations), it is likely that the phenotypic differences are caused by differential modification of the PS1 hypomorphic allele used in this study. While the authors clearly show that both the Notch-severe and Notch-mild animals exhibit the same defect in AβPP g-secretase cleavage, the Notch assay and the AβPP assay were done using different tissues (fibroblasts vs. brains) and at different ages (post-natal day 6 versus adult). As embryonic expression of PS1 is a critical factor for its...  Read more

Using genetic cross-breeding, Rozmahel et al. presented a novel finding that the presenilin 1 (PS1) developmental activity could be modified by genes located on the distal arm of chromosome 1. They further showed that one of the modifiers could be nicastrin, as the polymorphisms between 129 and C57BL/6J strains result in two missense substitutions in the nicastrin protein.
It would be very interesting to see how the two missense substitutions modify the functions of nicastrin and presenilins. Since the PS1 null phenotype does not seem to be affected by genetic background (our unpublished observations), it is likely that the phenotypic differences are caused by differential modification of the PS1 hypomorphic allele used in this study. While the authors clearly show that both the Notch-severe and Notch-mild animals exhibit the same defect in AβPP g-secretase cleavage, the Notch assay and the AβPP assay were done using different tissues (fibroblasts vs. brains) and at different ages (post-natal day 6 versus adult). As embryonic expression of PS1 is a critical factor for its notch and developmental activity (Qian et al., Neuron, 20: 611-617, 1998), it can be argued that the differential effect of the modifier on notch and AβPP could be due to the temporal-restricted modification of the hypomorphic PS1 allele. Therefore, absent a mechanism for the modifier, the author’s statement that “these results indicate that Notch S3-site cleavage and APP g-site cleavage are distinct presenilin-dependent processes” warrants further investigation.

View all comments by Hui Zheng

The results of this paper are very interesting and emphasize the critical role of nicastrin in PS-dependent γ-secretase activity. The problem is that the genetic findings are interpreted without adequate consideration of the biochemistry of γ-secretase. The principal conclusion is that the γ-secretase cleavage of AβPP and the S3 cleavage of Notch are distinct PS-dependent processes. In the discussion, emphasis is placed on the different sites of cleavage: in the middle of the transmembrane domain for AβPP and near the cytosolic edge for Notch. This apparent difference, however, has been resolved by the recent findings that AβPP is also cleaved near the cytosolic edge and Notch is also cleaved in the middle of their respective transmembrane domains. In other words, the transmembrane domains of these proteins are cleaved at least twice: a γ-secretase cleavage in the middle and an S3 (also called ε) cleavage near the cytosolic edge. These cuts are all PS-dependent. Moreover, in our hands all these cuts occur even after co-immunoprecipitation using PS-specific antibodies from...  Read more

The results of this paper are very interesting and emphasize the critical role of nicastrin in PS-dependent γ-secretase activity. The problem is that the genetic findings are interpreted without adequate consideration of the biochemistry of γ-secretase. The principal conclusion is that the γ-secretase cleavage of AβPP and the S3 cleavage of Notch are distinct PS-dependent processes. In the discussion, emphasis is placed on the different sites of cleavage: in the middle of the transmembrane domain for AβPP and near the cytosolic edge for Notch. This apparent difference, however, has been resolved by the recent findings that AβPP is also cleaved near the cytosolic edge and Notch is also cleaved in the middle of their respective transmembrane domains. In other words, the transmembrane domains of these proteins are cleaved at least twice: a γ-secretase cleavage in the middle and an S3 (also called ε) cleavage near the cytosolic edge. These cuts are all PS-dependent. Moreover, in our hands all these cuts occur even after co-immunoprecipitation using PS-specific antibodies from solublized membrane preparations. Unfortunately, the analysis of the AβPP and Notch proteolytic products in the present study is incomplete: Aβ is examined, but not the AICD; NICD is examined but not the Notch counterpart of Aβ. To conclude that the nicastrin mutations affect Notch and AβPP differentially, one must analyze both cleavage products generated from both Notch and AβPP.

But even assuming that there is indeed differential processing, this still does not mean that the γ-secretase cleavage of AβPP and the S3 cleavage of Notch are distinct PS-dependent processes. There is a perfectly reasonable biochemical explanation that is also consistent with other biochemical observation pointed out above: These nicastrin mutations (and also some reported PS mutations) lead to alterations in the substrate binding site(s) that have differential effects on the ability of the two substrates to interact with the protease.

View all comments by Michael Wolfe