A report in the January 24 online Journal of Biological Chemistry indicates that the Notch ligands Delta and Jagged are also substrates for γ-secretase, the intramembrane aspartyl protease responsible for processing of Notch and AβPP.

Like Notch, Delta and Jagged are transmembrane proteins. Delta, also like Notch, can be cleaved by a metalloprotease that removes the extracellular domain and leaves a mostly intracellular C-terminal fragment anchored to the cell membrane. Now, Takeshi Ikeuchi and Sangram Sisodia at the University of Chicago, using a version of Delta engineered with a traceable tag, shows that a slightly smaller version of the C-terminal fragment, which the authors dub D-CTF2, shows up in cultured cells. The size of D-CTF2 suggests that the C-terminal fragment has been cleaved at or close to the cell membrane. This alone does not necessarily implicate γ-secretase in the process, however, when Ikeuchi added a potent γ-secretase inhibitor to the cells, D-CTF2 no longer appeared, nor did it appear in cells expressing inactive γ-secretase. Similar experiments revealed the presence of a Jagged C-terminal fragment (J-CTF1) that is cleaved by γ-secretase to a smaller form (J-CTF2).

But what is the role, if any, of D-CTF2? Could it be translocated to the nucleus to activate transcription, as Notch and AβPP intracellular domains may (see ARF related news story)? To test this hypothesis, Ikeuchi and Sisodia coupled the C-terminal end of Delta to a transcriptional activator and expressed it in cells containing a luciferase gene coupled to the activator's promoter; translocation of the hybrid Delta to the nucleus should be detected by the luciferase's bioluminescence. Sure enough, cells with the hybrid had 70 times more bioluminescence than cells with the luciferase gene alone, indicating that D-CTF2 can indeed travel into the nucleus. A similar fate may await J-CTF2, as the intracellular domains of both Delta and Jagged have putative nuclear localization signals, the cellular equivalent of an entry visa.

These findings lead the authors to speculate that Delta and Jagged may deliver a double whammy to the nucleus-activation of transcription via the Notch pathway, and a similar activation via their own intracellular domains, the targets of which remain to be found.—Tom Fagan

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  1. The data presented here are similar to that presented at recent meetings by Taylor Kimberly, Dennis Selkoe and colleagues. Collectively these studies add to the growing list of gamma-substrates. The finding that Notch ligands may be substrates for gamma-secretase is quite interesting given the essential role of PS/gamma-secretase in Notch signaling. A concern with any studies reporting novel gamma-secretase substrates that was echoed by Bart De Strooper at the Stockholm meeting this past summer is that the evidence for cleavage in many cases is only from transfected cell studies. For all candidate gamma-substrates studies, endogenous levels of substrates will be needed to confirm that these are normal events. Subsequent studies will be needed to determine whether the gamma-cleavages of novel substrates are meaningful from a physiological perspective. Preliminary reports such as this are valuable as they set the stage for follow up studies.

    View all comments by Todd E. Golde

References

News Citations

  1. Does the Intracellular AβPP Fragment Regulate Calcium?

Further Reading

Primary Papers

  1. . The Notch ligands, Delta1 and Jagged2, are substrates for presenilin-dependent "gamma-secretase" cleavage. J Biol Chem. 2003 Mar 7;278(10):7751-4. PubMed.