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

Abstract

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.

Differences in the spatiotemporal expression and epistatic gene regulation of the mesodiencephalic dopaminergic precursor markerPITX3during chicken and mouse development

“…Pitx3 is first detected at E11.5 in mouse brain (Smidt et al, 1997), and we confirmed restricted Pitx3 expression in ventral midline of the mouse mesencephalon between E11.5 and E14.5 (Fig. S3A,A′,C,C′,E,E′; data not shown; n=3 per stage; Hoekstra et al, 2013). However, our expression analysis further revealed two additional Pitx3 + domains in the di-and mesencephalon.…”

“…Pitx3 expression initiates in the ventral mesencephalon and later extends into the caudal ventral diencephalon and it is restricted to postmitotic mdDA precursors and neurons located in the MZ of mesencephalic tegmentum (this work; Hoekstra et al, 2013). The expression of this gene initiates only shortly before or just after Th expression at E11.5 in mouse (Maxwell et al, 2005;Smidt et al, 1997;Zhao et al, 2004).…”

Mest/Peg1 Is Essential for the Development and Maintenance of a SNc Neuronal Subset

“…During development these neurons make-up the different molecular subsets of the mdDA neuronal population, dependent on a unique set of signaling cascades and transcription factors (Smits et al, 2006; Di Salvio et al, 2010; Jacobs et al, 2011; Hoekstra et al, 2013; Smits et al, 2013; Veenvliet et al, 2013). The existence of these subsets is particularly apparent during Parkinson’s Disease (PD), in which neurons of the SNc show a specific vulnerability for neuronal degeneration, whereas neurons of the VTA remain relatively intact (Barzilai and Melamed, 2003).…”

“…It consists of different molecular subsets that are specified during embryogenesis by a unique set of transcription factors and signaling cascades (Smits et al, 2006, 2013; Di Salvio et al, 2010; Jacobs et al, 2011; Hoekstra et al, 2013; Veenvliet et al, 2013). The presence of different molecular subsets in the mdDA neuronal population is nicely illustrated by the selective loss of mdDA neurons of the substantia nigra (SNc) during Parkinson’s Disease (PD).…”

Establishing diversity in the dopaminergic system

“…3). In mouse, LMX1A and LMX1B are partially redundant: only inactivation of both Lmx1a and Lmx1b results in the almost complete loss of MbDNs, while MbDNs are still generated in Lmx1a or Lmx1b single knock‐outs, albeit in reduced numbers [55,86,87] [28,56,57]. Surprisingly, the complete inactivation of Lmx1a or Lmx1b seems to have distinct effects on medial and lateral MbDN progenitors.…”