1. The mechanism of apoptosis induction through CD95.

The Peter lab initially described that upon induction of apoptosis CD95 recruits the DD and DED containing adapter molecule FADD and the DED containing cysteine protease caspase-8, caspase-10 and the caspase-8 regulator c-FLIP to the activated receptor forming the death-inducing signaling complex (DISC) (1-4). The DISC was the first multi-protein signaling complex described to regulate apoptosis. Another such complex that was identified is the apoptosome.

Model to illustrate the dual function of CD95. When aggregation and internalization of CD95 is not achieved or prevented CD95 apoptosis signaling is blocked and nonapoptotic signaling is activated which in cancer cells can promote tumor growth and/or progression.

Binding of the proenzyme procaspase-8 to the DISC results in its activation and the release of active caspase-8 into the cytoplasm where it can cleave a number death substrates including caspase-3, BID and proteins of the cytoskeleton such as plectin (5-7). Recently the Peter lab found that upon stimulation by CD95L CD95 internalizes in an actin dependent fashion (8,9). Interestingly in Type I cells most of the DISC forms after the stimulated receptor has reach an endosomal compartment (10) (Figure). When internalization is blocked CD95 does not mediate apoptosis but can still fully activate nonapoptotic signaling pathways (10) (Figure). It was found that a tyrosine phosphorylation site at position 291 of CD95 is critical for CD95 to internalize (10). More recently the Peter lab discovered a second posttranslational modification that regulates CD95 signaling. CD95 was found to be palmitoylated on cysteine 199 (11). Palmitoylation was found to be required for the receptor to form very high molecular weight complexes that were named the hiDISC (11,12) (see Figure). Similar to the tyrosine phosphorylation site palmitoylation of CD95 was recently found to be critical for CD95 to signal apoptosis and blocking palmitoylation does not inhibit CD95's activity to activate nonapoptotic signaling. Future studies are aimed at elucidating the role of the posttranslational modifications of CD95 as signaling switches between apoptosis and other pathways.

References

1.	Kischkel, F.C., Hellbardt, S., Behrmann, I., Germer, M., Pawlita, M., Krammer, P.H. and Peter, M.E. (1995) Cytotoxicity-dependent APO-1(Fas/CD95)-associated proteins form a death-inducing signalling complex (DISC) with the receptor. EMBO J., 14, 5579-5588.
2.	Muzio, M., Chinnaiyan, A.M., Kischkel, F.C., O' Rourke, K., Shevchenko, A., Scaffidi, C., Zhang, M., Ni, J., Gentz, R., Mann, M., Krammer, P.H., Peter, M.E.* and Dixit, V.M.* (1996) FLICE, a novel FADD-homologous ICE/CED-3-like protease, is recruited to the CD95 (Fas/APO-1) death-inducing signaling complex (DISC), Cell, 85, 817-827. * Authors share senior authorship. Voted by "Science" to be one of the top 10 discoveries of 1996. 3rd most cited paper in 1997.
3.	Peter, M.E. and Krammer, P.H. (2003) The CD95 death-inducing signaling complex and beyond. Cell Death Differ, 10, 26-35.
4.	Chang, D.W., Xing,, Z., Pan, Y., Algeciras-Schimnich, A., Barnhart, B.C.,Yaish-Ohad, S., Peter, M. E. and Yang, X. (2002) c-FLIPL is a dual function regulator for caspase-8 activation and CD95 (APO-1/Fas-mediated apoptosis. EMBO J., 21: 3704-3714.
5.	Medema, J.P., Scaffidi, C., Kischkel, F.C., Shevchenko, A., Mann, M., Krammer, P.H. and Peter, M.E. (1997) FLICE is activated by association with the CD95 death-inducing signaling complex (DISC), EMBO J., 16, 2794-2804.
6.	Chang, D.W., Xing, Z., Capacio, V., Peter, M.E and Yang, X. (2003) Interdimer processing mechanism of procaspase-8 activation. EMBO J., 22, 4132-4142.
7.	Stegh, A.H., Herrmann, H., Lampel, S., Weisenberger, D., Andrä, K., Seper, M., Wiche, G., Krammer, P.H. and Peter, M.E. (2000) Identification of the cytolinker plectin as a major early in vivo substrate for caspase-8 during CD95 and TNF-receptor mediated apoptosis. Mol. Cell. Biol., 20, 5665-5679.
8.	Algeciras-Schimnich, A., Shen, L., Barnhart, B.C., Murmann, A.E., Burkhardt, J and Peter, M.E. (2002) Molecular ordering of the initial signaling events of CD95. Mol. Cell. Biol. 22, 207-220.
9.	Algeciras-Schimnich A. and Peter, M.E. (2003) Actin dependent CD95 internalization is specific for Type I cells. FEBS Lett., 546, 185-188.
10.	Lee, K.-H., Feig, C., Tchikov, V., Schickel, R., Hallas, C., Schuetze, S., Peter, M.E.* and Chan, A.C.* (2006) The role of receptor internalization in CD95 signaling. EMBO J., 25, 1009-1023. * shared senior authorship. Highlighted in Nature Immunol. 7, 373 (2006) as "CD95 internalization".
11.	Feig, C., Tchikov, V., Schütze, S and Peter, M.E. (2007) Palmitoylation of CD95 facilitates formation of SDS-stable receptor aggregates that initiate apoptosis signaling. EMBO J., 26, 221-231. Highlighted in Nature Immunol. 8, 129 (2007) as "Palmitoylated Fas".
12.	Feig, C and Peter, M.E. (2008) Experimental approaches to detect and study the palmitoylated CD95 high-molecular weight death-inducing signaling complex (hiDISC). Methods in Ezymology, in press.