20 May 2005. Dogma holds that presenilins in the γ-secretase complex beget amyloid-β (Aβ) peptides by cleaving the membrane-bound β amyloid precursor protein (AβPP). But new results show there is (literally) another piece to the story. That piece is the AβPP intracellular domain (AICD), the cytosolic fragment of AβPP left after cleavage, which turns out to have the surprising ability to promote the destruction of Aβ. In a paper appearing in yesterday’s Neuron, Frederic Checler from Valbonne, France, and a multinational cohort of collaborators show that AICD activates transcription of the gene for the Aβ-degrading enzyme neprilysin. The ability of presenilins to coordinately affect Aβ production and degradation provides a neat physiologic mechanism for the tight regulation of Aβ levels, and may offer additional entry points for therapies that seek to lower those levels.
The starting point for first author Raphaelle Pardossi-Piquard and collaborators from France, Canada, Germany, Italy, and the US was an observation that fibroblasts or blastocysts from PS1 and PS2 double knockout mice failed to degrade Aβ peptides. From there, in an extensive series of well-controlled biochemical experiments, the researchers demonstrated that the cells specifically lacked neprilysin, but not other known Aβ proteases including insulin-degrading enzyme, and endothelin-converting enzyme. Transfecting the PS-/- cells with neprilysin fully restored Aβ degrading activity. The loss of neprilysin activity was accompanied by loss of neprilysin protein and mRNA, suggesting that somehow PS expression was regulating the transcription of the neprilysin gene. That regulation required γ-secretase activity, since several γ-secretase inhibitors mimicked the PS-/- phenotype in fibroblasts and in cultured neurons.
These results focused the researchers' interest on the AICD, since NICD, the intracellular domain of another γ-secretase substrate, Notch, had been shown to act as a transcription factor. Sure enough, when Pardossi-Piquard and colleagues expressed either a 50- or a 59-amino-acid AICD fragment in PS-/- cells, the cells regained neprilysin expression. The scientists went on to show that AICD increased activity of the neprilysin promoter in transactivation assays, that AICD could bind directly to the promoter DNA in mobility shift assays, and that it regulated neprilysin gene transcription via a pathway that involved the adaptor protein Fe65 and the histone acetytransferaseTip60, both proteins that were previously known to interact with AICD (see ARF related news story).
To show that endogenous AICD regulated neprilysin levels, the researchers checked enzyme activity in APP-deficient fibroblasts and in brain from APP knockout mice. In both cases, the investigators attained the expected result of decreased neprilysin activity. But activity was not null, which led them to check if intracellular domains from the APP-like proteins APLP1 and APLP2 also regulated neprilysin. They did, but Notch, e-cadherin, and n-cadherin intracellular fragments did not.
Finally, Pardossi-Piquard et al. determined that neprilysin expression and activity were higher in brain tissue from people with familial AD due to PS mutations compared to control tissue or brain from sporadic AD cases. This result is fully consistent with a physiologic role of PS and γ-secretase activity in neprilysin regulation.
“The experiments described here do more than simply confirm the previously and widely held suspicion that AICD, like NICD, might act as a signaling molecule involved in transcriptional activation,” the authors write. Instead, the demonstration that one product (AICD) of γ-secretase cleavage determines the lifetime of the other product (Aβ) reveals “an elegant and unusual method by which Aβ levels are controlled.”
One ramification of this elegant linkage of γ-secretase and neprilysin activity is the possibility that γ-secretase inhibition could be self-defeating if inhibitors adversely affect degradation of Aβ. But on the other hand, (and this work proves there is always an other hand), since increasing neprilysin activity is a proven way to decrease brain Aβ, (Leissring et al., 2003), the results could point to a new way of lowering Aβ independently of γ-secretase using AICD or mimics to boost neprilysin levels.—Pat McCaffrey.
Pardossi-Piquard R, Petit A, Kawarai T, Sunyach C, Alves da Costa C, Vincent B, Ring S, D’Adamio L, Shen J, Muller U, Hyslop PS, Checler F. Presenilin-Dependent Transcriptional Control of the Abeta Degrading Enzyme Neprilysin by Intracellular Domains of betaAPP and APLP. Neuron. 2005 May 19;46:541-554. Abstract