Working independently, two research groups-one led by Huaxi Xu at the Fisher Center for Alzheimer's Disease and Rockefeller University, New York, and the other by Peter St. George-Hyslop at the University of Toronto-strengthen the case for the involvement of APH-1 and PEN-2 in the γ-secretase complex.

First author Yongjun Gu and colleagues, working with St. George-Hyslop, used antibodies raised against the human analogue of APH-1 to identify where this protein resides in the cell and with what proteins it interacts. The antibody detected APH-1 in the endoplasmic reticulum and Golgi apparatus, two subcellular organelles where the presenilin complex assembles and matures. When the authors used the antibody to immunoprecipitate APH-1 from cell lysates, they found in the precipitate, PS1, PS2, N-terminal fragments of these two proteins, and nicastrin, all previously identified components of the complex, suggesting that APH-1 is a bona fide member of the presenilin club.

But what of its function? Interestingly, when Gu et al. immunoprecipitated APH-1 from cells expressing a mutant form of nicastrin that fails to mature, they detected immature nicastrin, but not PS1 in the precipitate, suggesting that mature nicastrin is required for APH-1 to bind the complex. Prior work on fruit flies has shown that loss of APH-1 reduces γ-secretase cleavage of AβPP (see Francis et al., 2002). St George-Hyslop and colleagues suggest that APH-1 may play a role in the maturation of the PS complex, perhaps by acting as a scaffold.

Under the direction of Huaxi Xu, Weijie Luo and colleagues investigated the role of the recently identified PEN-2, or presenilin enhancer-2 (see ARF related news story). When the authors used small interfering RNAs to silence PEN-2 expression in mammalian cells, they found a dramatic increase in the level of full-length PS1 and a reduction in the level of PS1 C-terminal fragments (PS1 C-terminal fragment); presenilin is normally proteolytically cleaved into smaller fragments that go on to form the γ-secretase complex. This result confirms a report from Christian Haas's laboratory, which showed that PEN-2 RNAi causes a dramatic reduction in levels of PS1 C-terminal fragment and a concomitant loss of the large PS1 complex (see Steiner et al., 2002).

Taken together, these experiments suggest that PEN-2 is necessary for proper processing of PS1. In support of this hypothesis, Luo et al. found that overexpression of PEN-2 enhances the proteolysis of full-length presenilin, leading to a dramatic increase of presenilin N-terminal fragments.

In a similar series of experiments the authors found that silencing aph-1 had little or no effect on PS1 processing. However, the accumulation of PS1 in cells lacking PEN-2 was reversed if aph-1 was also silenced. The authors suggest, therefore, that APH-1 stabilizes full length PS1, protecting it from rapid degradation. When Luo et al. overexpressed APH-1 they found it appeared to enhance the proteolytic effect of overexpressed PEN-2, and they comment that this may be explained by APH-1 also stabilizing the cleaved PS1 fragments.

Together the two papers seem to point to a scaffolding or protective role for APH-1, but a more active proteolytic role for PEN-2.-Tom Fagan.

References:
Gu Y, Chen F, Sanjo B, Kawarai T, Hasegawa H, Duthie M, Li W, Ruan X, Luthra A, Mount HTJ, Tandon A, Fraser PE, St George-Hyslop P. APH-1 interacts with mature and immature forms of presenilin and nicastrin and may play a role in maturation of presenilin-nicastrin complexes. J. Biological Chem. 2002 Dec 5.Abstract

Luo W, Wang H, Li H, Kim BS, Shah S, Lee H-J, Thinakaran G, Kim T-W, Yu G, Xu H. PEN-2 and APH-1 coordinately regulate proteolytic processing of presenilin 1. J. Biological Chem. 2003 January 8.Abstract