Wnt/beta-catenin signaling in midbrain dopaminergic neurons

Abstract

Parkinson's disease (PD) is caused by a progressive degeneration of the dopaminergic (DA) neurons of the substantia nigra. However, the mechanisms underlying the degeneration remain elusive and there is no cure for P1) at present. Cell replacement therapy (CRT), based on the transplantation of new DA neurons into P1) patients' brains, is a promising treatments strategy and it requires in depth knowledge of the signals regulating the generation of DA neurons during embryogenesis.

The Writ signaling pathway regulates neural patterning, cell fate determination, proliferation, differentiation, and neuronal maturation during vertebrate embryogenesis. In this thesis, evidence is presented that Wnt/beta-catenin signaling is also important during dopaminergic (DA) neuron development. Treatment of ventral midbrain (VM) precursor cultures with purified Wnt5a increases DA differentiation. Similarly, inhibition of GSK-30 or direct overexpression of beta-catenin increases the conversion of Nurr1+ precursors into DA neurons. Expression profiling of Writ components during midbrain development revealed that Frizzled (Fz) 9 is absent in newborn DA neurons while highly expressed in DA precursors. Upon closer examination we found that Fz9 inhibits Wnt5a signaling in DA cells, suggesting that the effects of Wnt5a during DA development require low levels of FA.

A widespread clinical application of CRT has been hindered by an inadequate availability of fetal tissue and poor graft survival within the host brain. Data presented herein demonstrates that treatment of VM precursor cultures with the JNK inhibitor SP600125 causes a great increase in the number of DA neurons, through reduced apoptotic signaling. The observed survival effect exhibits a degree of specificity for the DA lineage since the total neuronal pool is not significantly altered. When used upon transplantation into a rodent model of PD, SP600125 doubles the amount of surviving DA neurons. Combined, these results indicate that inhibition of JNK signaling can significantly reduce the number of DA neurons needed for grafting and improve the survival of grafted within the host brain, thereby fortifying CRT as a viable option in the treatment of PD.

Finally, the E3 ubiquitin ligase Parkin is identified as a novel regulator of Wnt/beta-catenin signaling and accordingly, beta-catenin levels are increased in parkin null animals. Stabilization of beta-catenin in differentiated primary VM neurons initially results in increased levels of cyclin E and proliferation, followed by increased levels of cleaved PARP and a loss of DA neurons. These findings suggest that Parkin could serve to protect DA neurons against Wnt/beta-catenin-induced toxicity. In summary, this thesis aims at extending our understanding of Wnt/beta-catenin signaling in midbrain dopamine neurons.