Dopaminerge Stammzellen im rostralen Migrationsstrom der Maus

Parkinson's disease (PD) is caused by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), which results in dopamine (DA) depletion in the brain of affected patients. Today's therapy options are mostly symptomatic and limited to pharmacological replacement therapy of DA. Because medication is associated with several side-effects and efficacy of the treatment subsides with time, alternative therapy options are studied since many years. Especially transplantation of endogenous and exogenous dopaminergic stem cells as a possible cure for PD is increasingly under investigation. The adult mammalian brain contains only two regions that maintain the lifelong capacity to generate new neurons, a process which is called neurogenesis. The first region is the subgranular layer (SGZ) which is located in the dentate gyrus of the hippocampus. The second region is the subventricular zone (SVZ) adjacent to the walls of the lateral ventricles of the telencephalon. Neurons deriving from neural stem cells (NSCs) of the SVZ migrate along the rostral migratory stream (RMS) towards the olfactory bulb (BO). After reaching the BO these neurons differentiate into mature interneurons before integrating into the granular (GCL) or periglomerular cell layer (PGL). Approximately 40% of the newly integrated neurons of the PGL show a dopaminergic phenotype. The aim of the present study was to investigate the localization of the dopaminergic NSCs in the SVZ-RMS-BO-system. It is of special interest to find the exact position of these neurons because they could be used for transplantation projects in clinical studies. To elucidate this question we used a mouse model consisting of 11 animals and implanted a physical barrier (PB) to mechanically interrupt the RMS. Depletion of neural progenitors in the SVZ and RMS was achieved by a continuous infusion of AraC (cytosine β-D-arabinofuranoside) administered for one week. Subsequently newly proliferating neurons were labeled by BrdU (5-bromo-2'- deoxyuridine), an analogue of thymidine. The experimental group consisted of 8 animals which were sacrificed 55 (n=4) and 105 days (n=4) after stopping the AraC application. The three animals of the control group received neither a surgery for PB implantation nor AraC treatment. They only obtained BrdU injections and were sacrificed on day 105. For further analysis the mouse brains were removed from the skull. The brain tissue was processed and immunhistochemically stained. The proper function of the PB, as required for the experiment, was confirmed by investigation of the SVZ. As expected after depletion of the neuroblasts neurogenesis was regenerating after treatment, which was confirmed by BrdU immunostaining. Furthermore a distinct accumulation of neuroblasts in the SVZ on the interrupted side indicated a proper function of the PB. Interestingly, the BrdU staining of the BO on the interrupted side showed a significant reduction of new born neurons in the GCL, but not in the PGL. This result clearly implies that NSCs, which produce interneurons destined for the GCL are located in the SVZ, whereas NSCs which provide interneurons for the PGL (including dopaminergic interneurons) must be located in the RMS. With the help of a BrdU/NeuN-double staining of the PGL it was confirmed that the cell type of the new born cells were mainly neurons instead of reactive glial cells. Furthermore, various stainings, especially the TH-staining of the PGL validated a constant level of dopaminergic neurons. To the best of our knowledge, this is the first work detecting dopaminergic stem cells in the RMS of a mouse model by mechanical interruption of the RMS. These findings are congruent with the results of different scientific groups and support the hypothesis that the SVZ-RMS-BO system is a complex proliferative zone. In addition, it becomes obvious that NSCs are an inhomogenous cell population in which NSCs of distinct regions produce different subtypes of neurons. This diverse character of NSCs is described as regionalization by different authors. Stem cell therapy of the central nervous system is a promising new approach for the treatment of PD as well as for other neurodegenerative and neurooncological diseases. Previously described sources of dopaminergic stem cells comprise several disadvantages and clinical limitations. The extraction of dopaminergic NSCs from the RMS could be a promising alternative to overcome current objections. First preliminary studies on rodents and humans were already successfully accomplished.