Generation of dopamine neurons in vivo and from embryonic stem cells in vitro

Mesencephalic dopamine (mesDA) neurons degenerate in patients with Parkinson s disease. In order to develop new treatment strategies for rescuing or replacing degenerating mesDA neurons, an improved understanding of the development and survival of these neurons in the normal brain is required. In paper I we identify two key players, Lmx1a and Msx1, implicated in the specification of mesDA neurons. Lmx1a and Msx1 are important for ensuring the correct mesDA neuron differentiation cascade by suppressing alternative cell fates, promoting neurogenesis and inducing the generation of mesDA neurons. In papers I and II we provide evidence that over-expression of Lmx1a both in mouse and human embryonic stem cells leads to an efficient generation of authentic mesDA neurons. These in vitro engineered mesDA neurons might, in the future, be used as a cell source in cell replacement therapy for patients with Parkinson s disease. However, to ensure a safe treatment, the use of embryonic stem cells depends on avoiding adverse effects such as uncontrolled cell growth. In papers II and III we show that an enrichment for embryonic stem cellderived progenitor cells or mature neurons, prior to grafting, removes unwanted dividing cells. Moreover, a higher number of mesDA neurons are present in the grafts after transplantation of sorted neurons, compared to progenitor cells. In paper IV, we show that ligands for the retinoic X receptor are present in the mesDA neuron domain in vivo, and that they mediate neuronal survival in primary mesDA neurons expressing both Nurr1 and the retinoic X receptor. These factors might be of interest as a potential treatment strategy to enhance survival of endogenous or transplanted mesDA neurons for patients with Parkinson s disease. The work presented in this thesis has increased the knowledge concerning the development of mesDA neurons and has helped to establish a protocol enabling the production of highly purified mesDA neurons. This knowledge can facilitate the establishment of new therapies for patients with Parkinson s disease, be a useful tool in drug screenings and help to identify additional factors important for the development, survival and maintenance of the mesDA system