Lmx1a Encodes a Rostral Set of Mesodiencephalic Dopaminergic Neurons Marked by the Wnt/B-Catenin Signaling Activator R-spondin 2

Recent developments in molecular programming of mesodiencephalic dopaminergic (mdDA) neurons have led to the identification of many transcription factors playing a role in mdDA specification. LIM homeodomain transcription factor Lmx1a is essential for chick mdDA development, and for the efficient differentiation of ES-cells towards a dopaminergic phenotype. In this study, we aimed towards a more detailed understanding of the subtle phenotype in Lmx1a-deficient (dreher) mice, by means of gene expression profiling. Transcriptome analysis was performed, to elucidate the exact molecular programming underlying the neuronal deficits after loss of Lmx1a. Subsequent expression analysis on brain sections, confirmed that Nurr1 is regulated by Lmx1a, and additional downstream targets were identified, like Pou4f1, Pbx1, Pitx2, C130021l20Rik, Calb2 and Rspo2. In line with a specific, rostral-lateral (prosomer 2/3) loss of expression of most of these genes during development, Nurr1 and C130021l20Rik were affected in the SNc of the mature mdDA system. Interestingly, this deficit was marked by the complete loss of the Wnt/b-catenin signaling activator Rspo2 in this domain. Subsequent analysis of Rspo2-/- embryos revealed affected mdDA neurons, partially phenocopying the Lmx1a mutant. To conclude, our study revealed that Lmx1a is essential for a rostral-lateral subset of the mdDA neuronal field, where it might serve a critical function in modulating proliferation and differentiation of mdDA progenitors through the regulation of the Wnt activator Rspo2.