Understanding Life and Death at CD95

The Crosstalk of Apoptotic and Non-Apoptotic Signaling in CD95 System

The mechanisms of CD95 (Fas/APO-1)-mediated extrinsic apoptotic pathway in cancer cells have been extensively studied. The majority of human cells express CD95, but not all these cells can induce extrinsic apoptosis. Accumulating evidence has shown that CD95 is a multifunctional protein, and its stimulation can also elicit non-apoptotic or even survival signals. It has become clear that under certain cellular contexts, due to the various checkpoints, CD95 activation can trigger both apoptotic and non-apoptotic signals. The crosstalk of death and survival signals may occur at different levels of signal transduction. The strength of the CD95 stimulation, initial levels of anti-apoptotic proteins, and posttranslational modifications of the core DISC components have been proposed to be the most important factors in the life/death decisions at CD95. Successful therapeutic targeting of CD95 signaling pathways will require a better understanding of the crosstalk between CD95-induced apoptotic and cell survival pathways. In this review, in order to gain a systematic understanding of the crosstalk between CD95-mediated apoptosis and non-apoptotic signaling, we will discuss these issues in a step-by-step way.

Quantification of CD95-induced apoptosis and NF-κB activation at the single cell level

CD95/Fas/APO-1 is a member of the death receptor (DR) family. Stimulation of CD95 leads to the induction of apoptosis as well as to NF-κB signaling. Crosstalk between these two pathways plays a central role in cell fate. Defects in the regulation of apoptosis and of NF-κB are connected to a number of chronic inflammatory diseases and cancer. For a better understanding of the life/death decisions in the cell and their contribution to disease progression, the development of new technologies is required. Using imaging flow cytometry we developed a method that enables a quantitative detection of different CD95 signaling pathways in the single cell. The important advantage of this method compared to other approaches is that it allows quantifying a large number of single cells undergoing apoptosis and NF-κB activation. This technology could provide new insights into the quantitative characterization of apoptosis and NF-κB at the single cell level and could be used for the quantitative network analysis in systems biology studies.

Systems biology of death receptor networks: live and let die

The extrinsic apoptotic pathway is initiated by death receptor activation. Death receptor activation leads to the formation of death receptor signaling platforms, resulting in the demolition of the cell. Despite the fact that death receptor-mediated apoptosis has been studied to a high level of detail, its quantitative regulation until recently has been poorly understood. This situation has dramatically changed in the last years. Creation of mathematical models of death receptor signaling led to an enormous progress in the quantitative understanding of the network regulation and provided fascinating insights into the mechanisms of apoptosis control. In the following sections, the models of the death receptor signaling and their biological implications will be addressed. Central attention will be given to the models of CD95/Fas/APO-1, an exemplified member of the death receptor signaling pathways. The CD95 death-inducing signaling complex (DISC) and regulation of CD95 DISC activity by its key inhibitor c-FLIP, have been vigorously investigated by modeling approaches, and therefore will be the major topic here. Furthermore, the non-linear dynamics of the DISC, positive feedback loops and bistability as well as stoichiometric switches in extrinsic apoptosis will be discussed. Collectively, this review gives a comprehensive view how the mathematical modeling supported by quantitative experimental approaches has provided a new understanding of the death receptor signaling network.