The intramembrane protease activity of γ-secretase is a prime target for therapeutic strategies aimed at treating Alzheimer disease by blocking amyloid β (Aβ) production (see ARF related Sorrento news story). But the biggest fly in the potential γ-secretase ointment is the fact that the enzyme cleaves a number of physiologically important substrates besides the amyloidogenic β amyloid precursor protein C-terminal fragment (CTF) (see ARF related news story). Now, a potential strategy—based on differences in membrane microdomain localization—for targeting Aβ cleavage while leaving the processing of proteins like Notch alone comes from findings reported in the May 10 J. Biol. Chem. by Gopal Thinakaran and colleagues at the University of Chicago.
Using density gradient fractionation, detergent solubilization, and cross-linking techniques, first author Kulandaivelu Vetrivel and coworkers determined the membrane location of γ-secretase and several of its substrates in cultured cells and in mouse brain (see ARF related news story). In these experiments, the researchers found most of the AβPP CTFs and γ-secretase colocalized in lipid rafts, cholesterol and sphingolipid-rich detergent-insoluble membrane domains thought to contain signaling complexes in cells (see ARF related news story). In contrast, the CTFs of other γ-secretase substrates including Notch1 and N-cadherin, Jagged2, and DCC were found in a different, detergent-soluble membrane domain. Though Vetrivel and colleagues found that active γ-secretase cleavage is not restricted to lipid rafts—the researchers detected enough detergent-soluble protease activity to account for the processing of substrates like Notch1 in non-raft membranes—these results show that most of the active γ-secretase and APP CTFs are likely sequestered away from other γ-secretase substrates. The distribution may be different in the embryonic brain, where the researchers found a significant amount of the γ-secretase proteins presenilin and nicastrin, as well as APP CTFs in the detergent-soluble membrane fraction.
This suggestion of a spatial separation of APP and Notch processing has potential implications in developing therapeutics aimed at reducing Aβ burden, the authors write. “By selectively targeting γ-secretase inhibitors to cholesterol- and sphingolipid-rich membrane domains, it may be possible to inhibit Aβ production without adverse effects on non-raft resident substrates,” they conclude.—Pat McCaffrey
- Vetrivel KS, Cheng H, Kim SH, Chen Y, Barnes NY, Parent AT, Sisodia SS, Thinakaran G. Spatial segregation of gamma-secretase and substrates in distinct membrane domains. J Biol Chem. 2005 Jul 8;280(27):25892-900. PubMed.