Early and localized expression of ion channels and ion channel subunits in the developing mesodiencephalic dopaminergic domain of the mouse embryo: P02--03

Dopamine (DA)-synthesizing neurons located in the mam-malian ventral midbrain and caudal forebrain are crucially involved in the control and modulation of voluntary move-ments, habit learning, rewarding/aversive behaviors and cognitive functions of the brain. The degeneration of these neurons leads to the characteristic motor symptoms of Par-kinson's Disease (PD), whereas their dysfunction is tightly associated with neuropsychiatric disorders such as addic-tion, schizophrenia, attention deficit hyperactivity disorder and depression. The etiology of these disorders, but in particular of the neuropsychiatric diseases, is meanwhile thought to have a strong neurodevelopmental component. The mesodiencephalic dopaminergic (mdDA) neurons arise from the ventral midline of the mesencephalon (midbrain) and caudal diencephalon starting at around day 10.5 to 11.5 (E10.5-11.5) of mouse embryonic development. Sev-eral signaling pathways are implicated in these early steps of mdDA neuron development, but the most important of all is the WNT/β-catenin signaling pathway. From E11.5 on-wards, this signaling pathway directs the specification of an mdDA cell fate in ventral midline progenitors and their correct differentiation particularly into a rostrolateral mdDA neuron subset. To identify novel targets of the WNT/β-caten-in signaling pathway in this context, we laser-microdissect-ed the WNT-responsive mdDA domain in the E12.5 ventral midbrain/caudal diencephalon of a WNT reporter (BAT-gal) mouse. We subsequently compared the transcriptome of these WNT-responsive mdDA cells to the transcriptome of non-WNT-responsive and non-mdDA cells from an adjacent domain in the ventrolateral BAT-gal midbrain using microar-rays. This approach enabled us to determine the transcripts that are preferentially expressed in the developing WNT-re-sponsive mdDA domain. Surprisingly, around 51% of the enriched transcripts encode proteins belonging to the ion channel and neurotransmitter receptor/transporter gene on-tology categories. In situ validation of some of these genes, including the voltage-gated K channel Kcnd3 (Kv4.3), the inwardly-rectifying K channel Kcnj6 (Girk2) and the volt-age-dependent Ca channel subunit Cacna1d (Cav1.3α1), revealed that they are selectively transcribed in the ven-tral midbrain/mdDA domain of the E12.5 mouse embryo, exhibiting in some cases a very localized expression pat-tern in what appears to be mdDA subdomains. All three ion channels/subunits are crucial for the normal (pacemaker) function of mdDA neurons, and have been implicated in the pathophysiology of PD. Because the neural connectivity of the mdDA neurons has not yet been established at this ear-ly developmental stage (E12.5), our data hint at other func-tions of these ion channels/ion channel subunits during early mdDA neuron development.