GLIOBLASTOM - Anni Hofmann Stiftung

The dual role of mesenchymal stem cells in gliomas: new therapies and therapeutic targets.

Prof. Dr. Rainer Glaß, Leiter der Neurochirurgischen Forschung, Klinik für Neurochirurgie, Klinikum der Universität München, Marchioninistr. 15 81377 München. Email: rainer.glass@med.uni-muenchen.de 

Report-period: 7 / 2014 until 3 / 2015.

Primary brain tumors (gliomas) comprise less than 2% of the worldwide cancer burden, but they are a leading cause for cancer-related death. Malignant gliomas like glioblastoma multiforme (GBM) are very heterogeneous tumors and contain a subpopulation of highly aggressive neoplastic cells, so-called glioma stem cells (GSC), which may largely be responsible for therapeutic resistance and tumour-relapse. GSCs constitute a promising target for glioma therapies if we can uncover ways to render these cells more sensitive to chemotherapeutics and blunt their capacity for initiating tumor recurrence.

Evidence suggests that the stem-like capacity of GSCs is modulated by the glioma-microenvironment. GSCs may not be targeted directly (do to their inherent therapeutic resistance), but indirectly e.g. through parenchymal cells like mesenchymal stem cells (MSCs), which accumulate in the glioma microenvironment. We and others found that MSC are common and abundant in gliomas, they continuously exchange information with glioma cells via soluble factors or communicate through the exchange of small vesicles (exosomes).

We observed that MSCs can induce differentiation effects in GSCs through the release of exosomes. The exosomal contribution of various molecular elements to the intercellular communication network is not well established, and is a fairly new concept with regards to tumor formation. Exosomes, biological nano-particles that are 50-100 nm in diameter, can contain RNAs, e.g. coding (mRNA) and non-coding (miRNAs) elements, which directly or indirectly modulate the signal transduction pathways in exosome-receiving cells.

To identify the molecular pathways which induce GSC-differentiation (thereby converting GSC into bulk glioma cells) we have performed genome wide expression arrays of human primary GBM cultures receiving MSC conditioned media (MSC-CM; maintained for three days under in vitro conditions lacking serum and containing soluble factors and exosomes liberated from MSC) over a time-course (0 hours, control-sample; 24 h, 48 h and 72 h stimulation with MSC-CM). Furthermore, we have performed a next-generation sequencing (NGS) analysis of the genetic elements in MSC-derived exosomes, in human primary GBM cells stimulated with exosomes and in a control group (unstimulated GBM cells). Currently we use bioinformatics to analyze our data from these different sets of experiments and to identify a unifying signaling pathway explaining the GSC-directed anti-tumorigenic effect of MSC-released exosomes. In addition, we have also advanced in identifying the molecules mediating the tumor supporting effects of glioma-associated MSC. In our new series of in vitro experiments we could observe that the pro-tumourigenic effects of MSC are mediated via soluble factors which predominantly support the viability of differentiated glioma cells maintained under serum-free culture conditions. With the combined expression arrays (as outlined above, determining the physiological effects of unfractionated MSC-CM and also specifically of the exosome fraction from MSC-CM) we have now obtained candidate signaling pathways in human primary GBM cells that are modulated by soluble factors from MSC-CM and regulate the viability and neoplastic properties of brain tumors. Overall, we have completed the work programme of work-packages-1 and -3 (and large parts of WP-2) and are currently deciphering endogenous, MSC-induced pathways to ablate GSCs (as mediated by MSC-derived exosomes) or promoting glioma-resistance (via factors from MSC-CM). Overall, we have widened the spectrum of experiments of our original proposal by taking up a study on exosomal transfer and we are advancing with our translational approach to target GBM, by establishing new tumor suppressors (the anti-tumor factors in can e.g. be applied as therapeutics) or by establishing new therapeutic targets (e.g. by pharmacologically interfering with the pro-tumor effect of MSC-released soluble factors).