Our research focusses on the molecular mechanisms that regulate human adult stem cells. We characterize human blood stem cells (hematopoietic stem and progenitor cells, HSC or HPC) derived from umbilical cord blood, bone marrow and mobilized peripheral blood, as well as mesenchymal stem cells (MSC) from bone marrow and adipose tissue. Using state of the art technology (e.g. flow cytometry, microarray technology, sequencing methods, western-blot, and confocal microscopy), we determine molecular characteristics and correlated these with various functional assays.

Figure 1: Stem cells from human bone marrow.

Since 2006, it is possible to redirect differentiated cells via transfection with specific transcription factors back into a pluripotent state – this is commonly referred to as “reprogramming”. These induced-pluripotent stem cells (iPSC) can be differentiated into all cell types of the human body. They reveal the same functional features as embryonic stem cells without being isolated from human embryos (For this finding Prof. Yamanaka was awarded the Nobel Prize in Physiology or Medicine 2012).  iPSC are an ideal model system to better understand mechanisms involved in differentiation, aging, and malignant transformation. On the long run, our investigations shall contribute to new and safe therapeutic options in regenerative medicine.

Figure 2: Classification of stem cells.

Characterization of hematopoietic stem cells

Hematopoietic stem cells (HSC) are successfully transplanted for more than 40 years. However, the molecular characteristics of HSC remain largely unknown. It is possible to enrich multipotent subpopulations by surface markers using flow cytometry (e.g. CD34+ CD38- cell fraction). Currently, we are analyzing specific genes and micro-RNA as well as epigenetic mechanisms that contribute to the regenerative potential of these cells.

Characterization of mesenchymal stem cells

Mesenchymal stem cells (MSC) are currently tested for broad range of diseases in clinical trials. For isolation and expansion of these cell preparations, it is necessary to cultivate MSC in vitro. Hereby, external parameters such as culture media, biomaterials, cell culture techniques, and replicative senescence have major impact on cellular composition and differentiation potential. Therefore, it is well conceivable that these parameters affect the therapeutic potential of MSC. Using gene expression analysis, micro-RNA profiling, sequencing methods, and epigenetics our group investigates molecular characteristics and the influence of specific culture conditions on MSC.

Figure 3: Mesenchymal stem cells.

Interaction of stem cells with their niche

Adult stem cells ensure lifelong regeneration of all our tissues. They are characterized by the ability to differentiate into different cells types. On the other hand, they reproduce multipotent stem cells to maintain to the stem cell pool. This dual function has to be tightly regulated according to the physiological needs by interaction of stem cells with their cellular microenvironment - the "stem cell niche". In case of HSC, the niche is located in the bone marrow. MSC are located in the bone marrow, too, and our research has supported the notion that MSC are a suitable model system to analyze interaction of HSC with a cellular microenvironment. Direct cell-cell contact seems to be of central importance for regulation of stem cell function. Therefore, we are analyzing the role of cell-adhesion proteins for podia formation, cell-cell adhesion, maintenance of differentiation potential and homing to the stem cell niche. The aim of these studies is generating optimal conditions for in vitro expansion of HSC without loss of stem cell function. In addition, we are analyzing epigenetic changes during culture and interaction of HSC with non-cellular surfaces.

Stem cells and biomaterials

Cell culture is commonly performed in commercially available cell culture flasks made of plastic (polystyrol). However, during the last years there is accumulating evidence that biomaterials, topography, and elasticity of the surface have major influence on cellular growth and differentiation capacity. Furthermore, mechanical stimulation can direct cellular differentiation, too. In cooperation with various institutes for material research in Aachen we are investigating the influence of these parameters on growth and differentiation of HSC, MSC, and iPSC.

The human body is as old as his stem cells

Regenerative potential of our tissues decays in the elderly and it is governed by tissue-specific stem cells – hence, it is assumed that the process of aging arises from a decay of the regenerative potential of adult stem cells. In our previous studies, we could show that gene expression profiles of MSC and HSC change upon aging. Furthermore, this process is associated with highly reproducible epigenetic modifications at specific sites in the genome. We could demonstrate that donor age can be estimated with a mean average deviation of only 5 years by analyzing DNA-methylation patterns at only three CpG sites. Further information on this method is available in a short movie (only available in German). Interestingly, these age-related molecular changes are almost entirely reversed by reprogramming into iPSC – thus these cells seem to be rejuvenated. We want to better understand the underlying mechanisms.

Figure 4: Colony of iPSC. These cells are rejuvenated on epigenetic level and do not reveal replicative senescence while in pluripotent state.

Epigenetic changes in malignant transformation and cancer

Cancer initiating cells are often derived from adult stem cells and they escape from replicative senescence. A central feature of these cells is high self-renewal potential – thus, there are many overlaps with the above mentioned research interests. We are currently analyzing how the DNA-methylation pattern is modified in different tumors – particularly in leukemia. Thereby, we identify epigenetic biomarkers to support diagnosis, prognosis and choice of therapeutic options.

This research was funded by

We would like explicitly to thank all patients who gave their consent to collect cord blood samples or other cell material and using these samples for our research projects anonymously! Without this confidence, our work would not be accomplishable.

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