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.

Reference:
Vetrivel KS, Cheng H, Kim SH, Chen Y, Barnes NY, Parent AT, Sisodia SS, Thinakaran G. Spatial segregation of γ-secretase and substrates in distinct membrane domains. J Biol Chem. 2005 May 10; [Epub ahead of print] Abstract