Research
Apoptosis is a fundamental process to regulate homeostasis of all tissues and to eliminate unwanted cells specifically in the immune system. Various parts of apoptosis signaling pathways have recently been characterized. Specifically in apoptosis pathways initiated by members of the death receptor family such as CD95 (APO-1/Fas) proteins that either contain a death domain (DD) or a death effector domain (DED) have been found to be essential. We are working on various aspects of the signal transduction of the apoptosis inducing death receptors with special emphasis on the CD95 receptor and the function of DED proteins. The following projects are currently studied:
1. Signaling through CD95.
We 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). 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. Subsequent studies demonstrated that caspase-8 has two ways to activate the downstream apoptosis machinery depending on the cell type. In Type I cells caspase-8 is activated at the death inducing signaling complex (DISC) in large quantities resulting in direct processing of caspase-3. This step is independent on mitochondria and cannot be blocked by overexpression of Bcl-2. In Type II cells the amount of active caspase-8 generated at the DISC is very small. Apoptosis in Type II cells depends on the apoptogenic activity of mitochondria and is characterized by activation of large quantities of caspase-3 and caspase-8 downstream of the mitochondria. Only in these cells overexpression of Bcl-2 or Bcl-xL blocks activation of both caspases and apoptosis. Recently we reported reported that CD95 clusters and internalizes in a caspase-8 and actin dependent fashion. However, this activity is restricted to Type I cells.
2. CD95 Type I and Type II cell represent mesenchymal and epithelial cells, respectively.
After years of controversy on the physiological function of CD95 ligand CD95L we recently demonstrated that the physiologic CD95 ligand is highly toxic to Type II cells but does not induce apoptosis on Type I cells. This allowed to type the 60 tumor cells of the NCI drug screening panel resulting in the discovery that Type I cells represent mesenchymal cells whereas Type II cells correspond to epithelial cells, respectively. It was also found that Type I and Type II tumor cells differ in their sensitivity to anti tumor drugs that target two major cytoskeleton systems. Type I cells are sensitive to all actin binding drugs whereas Type II cells are sensitive to tubulin binding drugs. These findings are relevant for activated T cells since it was previously shown that T cells differentiate from Type II to Type I during long-term T cell activation. Recent evidence suggest that the reason for the different signaling through CD95 in short-term versus long-term activated peripheral T-cells could be in the way the DISC is forming. Similar to the TNF-receptor I the DISC in Type II cells seems to form intracellularily.
3. The death receptor CD95 can act as a tumorigenic receptor on apoptosis resistant tumor cells through activation of NF-kB and MAP kinases.
Most tumor cells are resistant to CD95 mediated apoptosis but today not a single tumor has been reported that completely lacks CD95 expression. We recently showed that on CD95 apoptosis resistant tumor cells and CD95 sensitive Type I tumor cells when treated with soluble CD95L stimulation of CD95 results in activation of NF-kB and MAP kinases inducing increased motility and invasiveness of tumor cells. These novel findings underscore the novel role of death pathways and their components outside of apoptosis which are studied in my lab.
4. The role of the phosphorylation of FADD in cell cycle progression.
FADD is an adaptor molecule that allows recruitment of the initiator caspase-8 to the stimulated death receptor CD95 upon induction of apoptosis. However, it has been shown previously that FADD is also important for nonapoptotic processes such as cell cycle progression and T cell activation. The way FADD elicits these activities was unknown. We recently reported that it is the specific phosphorylation of FADD on serine 194 they identified previously that regulates its cell regulating activities pointing at the unknown kinase that phosphorylates FADD as important link between death receptor signaling and cell cycle progression. Once this kinase is identified the role of the phosphorylation of FADD in T cell activation can be determined.
5. DEDD as a platform protein to regulate activation of caspases.
As members of the death effector domain proteins we previously cloned DEDD and DEDD2. DEDD was found not only to be monoubiquitinated but also to be a strong ubiquitin binding protein. Ubiquitination and especially monoubiquitination has recently been recognized to be an important posttranslational modification found on proteins regulating apoptosis signaling. Monoubiquitination of DEDD seems to be important for DEDD to function as a platform protein to recruit and activate caspase-3 and caspase-9. This suggests that DEDD is required for the activation of caspase-9 which so far has been believed to be exclusively activated by the apoptosome, a multi-protein structure that comprises cytochrome c, Apaf-1 procaspase-9 and dATP. DEDD could therefore form an alternative apoptosome making it an important general apoptosis regulator.