Our group has pioneered the investigation of the non-apoptotic role of CD95(Martin-Villalba et al., 2013). Specifically, we have unraveled the molecular events leading to activation of tyrosine-kinase activities downstream of CD95 in various cellular system ranging from stem cells to neurons, immune and cancer cells(Corsini et al., 2009; Kleber et al., 2008; Letellier et al., 2010; Sancho-Martinez and Martin-Villalba, 2009; Teodorczyk et al., 2015; Zuliani et al., 2006). We find that non-apoptotic activities converge in tyrosine-kinase activity (Src, PI3K) but that the molecules linking CD95 to the activation of these kinases are cell-type-dependent.
CD95-mediated tyrosine kinase activity is used by developing neurons to increase the dendritic arbor complexity, which impacts brain functions such as mood or cognition(Zuliani et al., 2006). CD95 activity can be used to activate stem-cell repair and thereby rescue the cognitive impairment (Corsini et al., 2009). Its pro-inflammatory activities are involved in disease progression following spinal cord injury, stroke and Parkinson disease(Gao et al., 2015; Letellier et al., 2010). Finally, CD95 is crucial for survival of cancer cells and for activation of an EMT/Stemness-program in tumor cells that drives tumor growth and metastasis (Drachsler et al., 2016; Kleber et al., 2008; Teodorczyk et al., 2015; Teodorczyk and Martin-Villalba, 2010).
During brain development, multipotent neural progenitors undergo a tightly regulated differentiation process to ultimately produce the exquisite variety of neuronal subtypes that build the brain. In the adult brain, neurogenesis does not completely stop, and new neurons are continuously generated throughout life in two specific regions: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus. Since its discovery in the rodent brain in 1994, adult neurogenesis has been a field of intense research, fuelled by the hypothesis that stem cells are at the origin of brain tumors and the hope of enhancing the brain’s capacity to self-repair after injury or surgery.
We entered the field of stem cells, through the observation that in the adult CNS, CD95 is only expressed in neural stem cells (NSCs) but not in mature neurons. We found that this receptor is used by SGZ-NSCs as a survival and activation signal during homeostasis, and its deficiency led to cognitive impairments (Corsini et al., 2009). Most excitingly, this receptor was essential for activation of NSCs for brain repair following an ischemic insult (Corsini et al., 2009) (Covered in the same issue of Cell Stem Cell). This was followed by studies of Wnt activity in the adult stem cell compartment (Seib et al., 2013). In collaboration with the group of C. Niehrs, we generated two mouse lines with deletion of the Wnt antagonist DKK1 in the whole neural compartment or exclusively in adult stem cell compartment. We observed that DKK1 expression in NSCs increases with aging, and that its deletion counteracts the age-associated cognitive decline. Interestingly, the increase of hippocampal stem cells by deletion of DKK1 also counteracted the depression-like behavior in young animals. This study was covered in the same issue of Cell Stem Cell and by National Geographic (http://phenomena.nationalgeographic.com/2013/02/07/opening-the-black-box-of-neurogenesis/). In both studies we could measure the consequences of deletion of either CD95 or DKK1 but elucidating the underlying cellular mechanism was hampered by the limited amount of data that can be generated by animal studies. To circumvent this problem we started an ongoing collaboration with A. Marciniak-Czochra (Mathematical Analysis) on building a model of stem cell dynamics. A first model has already been published (Ziebell et al., 2014) and has set the basis for future studies on dynamics in tumor models, aging, and the exocrine pancreas.
While stem cells in the hippocampus are involved in cognition, stem cells in the ventricular zone are involved in olfaction and brain repair and represent the cell of origin of brain tumors. To study their behavior under homeostasis and following acute injury we developed a single cell transcriptomics strategy preceded by index-sorting (Llorens-Bobadilla et al., 2015). Analysis of the individual neural stem cell transcriptome uncovered stem cell heterogeneity imposed by four different states of activation and lineage restriction. In particular, we identified a novel state of dormancy defined by a very low level of translation and enhanced lipid biosynthesis and glycolysis –transcriptional programs that are gradually switched during stem cell activation. Studying single stem cell transcriptomes after ischemic damage, identified interferon-gamma as a new factor to activate stem cells for repair.
Corsini, N.S., Sancho-Martinez, I., Laudenklos, S., Glagow, D., Kumar, S., Letellier, E., Koch, P., Teodorczyk, M., Kleber, S., Klussmann, S., et al. (2009). The death receptor CD95 activates adult neural stem cells for working memory formation and brain repair. Cell Stem Cell 5, 178-190.
Drachsler, M., Kleber, S., Mateos, A., Volk, K., Mohr, N., Chen, S., Cirovic, B., Tuttenberg, J., Gieffers, C., Sykora, J., et al. (2016). CD95 maintains stem cell-like and non-classical EMT programs in primary human glioblastoma cells. Cell Death Dis 7, e2209.
Gao, L., Brenner, D., Llorens-Bobadilla, E., Saiz-Castro, G., Frank, T., Wieghofer, P., Hill, O., Thiemann, M., Karray, S., Prinz, M., et al. (2015). Infiltration of circulating myeloid cells through CD95L contributes to neurodegeneration in mice. JExpMed 212, 469-480.
Kleber, S., Sancho-Martinez, I., Wiestler, B., Beisel, A., Gieffers, C., Hill, O., Thiemann, M., Mueller, W., Sykora, J., Kuhn, A., et al. (2008). Yes and PI3K bind CD95 to signal invasion of glioblastoma. Cancer Cell 13, 235-248.
Letellier, E., Kumar, S., Sancho-Martinez, I., Krauth, S., Funke-Kaiser, A., Laudenklos, S., Konecki, K., Klussmann, S., Corsini, N.S., Kleber, S., et al. (2010). CD95-ligand on peripheral myeloid cells activates Syk kinase to trigger their recruitment to the inflammatory site. Immunity 32, 240-252.
Llorens-Bobadilla, E., Zhao, S., Baser, A., Saiz-Castro, G., Zwadlo, K., and Martin-Villalba, A. (2015). Single-Cell Transcriptomics Reveals a Population of Dormant Neural Stem Cells that Become Activated upon Brain Injury. Cell Stem Cell 17, 329-340.
Martin-Villalba, A., Llorens-Bobadilla, E., and Wollny, D. (2013). CD95 in cancer: tool or target? Trends MolMed.
Sancho-Martinez, I., and Martin-Villalba, A. (2009). Tyrosine phosphorylation and CD95: a FAScinating switch. Cell Cycle 8, 838-842.
Seib, D.R., Corsini, N.S., Ellwanger, K., Plaas, C., Mateos, A., Pitzer, C., Niehrs, C., Celikel, T., and Martin-Villalba, A. (2013). Loss of Dickkopf-1 restores neurogenesis in old age and counteracts cognitive decline. Cell Stem Cell 12, 204-214.
Teodorczyk, M., Kleber, S., Wollny, D., Sefrin, J.P., Aykut, B., Mateos, A., Herhaus, P., Sancho-Martinez, I., Hill, O., Gieffers, C., et al. (2015). CD95 promotes metastatic spread via Sck in pancreatic ductal adenocarcinoma. Cell DeathDiffer.
Teodorczyk, M., and Martin-Villalba, A. (2010). Sensing invasion: cell surface receptors driving spreading of glioblastoma. JCell Physiol 222, 1-10.
Wollny, D., Zhao, S., Everlien, I., Lun, X., Brunken, J., Brüne, D., Ziebell, F., Tabansky, I., Weichert, W., Marciniak-Czochra, A., et al. (2016). Single-Cell Analysis Uncovers Clonal Acinar Cell Heterogeneity in the Adult Pancreas. Developmental Cell.
Ziebell, F., Martin-Villalba, A., and Marciniak-Czochra, A. (2014). Mathematical modelling of adult hippocampal neurogenesis: effects of altered stem cell dynamics on cell counts and bromodeoxyuridine-labelled cells. JRSocInterface 11, 20140144.
Zuliani, C., Kleber, S., Klussmann, S., Wenger, T., Kenzelmann, M., Schreglmann, N., Martinez, A., del Rio, J.A., Soriano, E., Vodrazka, P., et al. (2006). Control of neuronal branching by the death receptor CD95 (Fas/Apo-1). Cell DeathDiffer 13, 31-40.