Stem cell/niche interactions in tissue homeostasis

Tissue and lineage specification of stem cells is believed to be controlled in a self-organizing, dynamically regulated process. This control relies on reciprocal signaling between stem cells and the local microenvironment and involves cues such as cell adhesion, growth and differentiation factors. The same signaling pathways that orchestrate embryonic development (e.g. fgf, tgfβ/bmp, shh, notch and Wnt pathways) are thought to regulate proliferation, provide positional information and induce cell fate specification of adult stem cells. We have shown recently that the Wnt pathway is one of the major players involved in the control of skin stem cell differentiation. Members of the Wnt gene family, of which the human genome contains almost twenty, are found in organisms ranging from nematode worms to mammals. Wnt genes encode secreted ligands which induce a conserved cytoplasmic signaling cascade controlling growth and differentiation (Fig. 1). The central player in this cascade is β‑catenin which transmits the Wnt signal into the nucleus where β‑catenin interacts with transcription factors and activates the transcription of target genes. Mutations in several genes of the pathway have been found to be causative in a variety of human cancers and result in constitutive signaling and gene activation. For instance, such genetic alterations are very abundant in human colon tumors.

Figure 1: Scheme of the different intracellular cascades triggered by Wnt signaling.
Molecules implicated in the canonical signaling cascade are shown in blue, molecules in the planar cell polarity (PCP) pathway in orange and proteins involved in calcium-mediated signaling in green. Depicted in grey are alternative interactions of ß‑catenin and APC in cell adhesion and cytoskeleton, which are not necessarily linked to Wnt signaling.

We currently characterize the mechanisms underlying the cross-talk of stem cell and the surrounding niche using large scale gene expression profiling. We were able to identify differentially expressed genes which are specific for the stem cell and niche population. Importantly, we have identified several genes to be expressed in a small population of stromal cells which are in close contact to bulge stem cells. We are the first to describe this stromal compartment in the skin which might contribute to the stem cell niche in order to define and localize skin stem cells in the bulge. We are analyzing the importance of this structure using in vivo assays which assess in which way stem cell function depends on these niche signals.

Tumor/stroma interactions in metastatic outgrowth

In a second line of research, we aim to identify molecules which are involved in early metastasis formation. Inefficient metastatic colonization is believed to reflect the inappropriate communication between tumor cells and the local microenvironment at a distant location in contrast to the primary site in which the tumor has initially evolved (Fig. 2). Only a few disseminated tumor cells might be able to cope with the new environment. This is reflected in experimental metastasis models where repeated passages of tumors have been used to select for cell clones with increasing metastatic potential.

Figure 2: Important steps during metastasis formation.
The metastatic cascade is initiated in the primary tumor by cells which gain migratory properties and invade into the surrounding stroma and the supplying vasculature. At the target organ, the cells actively extravasate or become trapped within the capillary network. Most of these cells will not be able to activate the surrounding stroma and either persist in a dormant state or die. Only very few tumor cells manage to adapt to the new environment, to activate the stroma and to form a micrometastasis. Few of these are able to expand after attraction of additional blood supply and grow into macrometastases (not shown).

Together with the bioinformatics group of Felix Naef, we have developed a novel method to simultaneously analyze the changes which occur in the tumor cells as well as the surrounding stroma during the selection process of metastatic initiation and progression. We utilize an in vivo, murine model system of liver metastasis, which is the major metastatic target site for a variety of human cancers such as colon, pancreatic, gastric or ovarian cancers. This model enables us to study early events in metastatic initiation and outgrowth which cannot be analyzed in patients. In this experimental setting all metastases are initiated by identical oncogenic precursor cells, therefore differences in successful metastatic progression is due to specific interactions between tumor and local host cells. In vivo selection by the microenvironment in the target organ will favor certain combinations of tumor and host cell features which are able to support metastatic colonization to varying extent. We have developed a new analysis method for transcriptional profiling of chimeric tissues. This system allows us to simultaneously analyze species-specific responses, and thus in our metastasis model also tissue-specific responses. This allows us to measure gene expression levels of the tumor (human) and immediately adjacent stroma (mouse) without physically separating mouse and human cells avoiding any degradation or expression changes which could occur during such an isolation procedure. This allows us to identify essential, reciprocal interactions at the tumor/host interface during metastatic colonization which will provide new possibilities of therapy.

Keywords

Stem cell biology, cancer stem cells, self-renewal vs. differentiation, signaling cascades, Wnt signaling