Dynamic temporal requirement of Wnt1 in midbrain dopamine neuron development.

Wnt1-expressing progenitors generate midbrain dopamine (MbDA) and cerebellum (Cb) neurons in distinct temporal windows and from spatially discrete progenitor domains. It has been shown that Wnt1 and Lmx1a participate in a cross-regulatory loop that is utilized during MbDA neuron development. However, Wnt1 expression dynamically changes over time and precedes that of Lmx1a. The spatial and temporal requirements of Wnt1 in development and specifically its requirement for MbDA neurons remain to be determined. To address these issues, we generated a conditional Wnt1 allele and temporally deleted Wnt1 coupled with genetic lineage analysis. Using this approach, we show that patterning of the midbrain (Mb) and Cb by Wnt1 occurs between the one-somite and the six- to eight-somite stages and is solely dependent on Wnt1 function in the Mb, but not in the Cb. Interestingly, an En1-derived domain persists after the early deletion of Wnt1 and mutant cells express OTX2. However, the En1-derived Wnt1-mutant domain does not contain LMX1a-expressing progenitors, and MbDA neurons are depleted. Thus, we demonstrate an early requirement of Wnt1 for all MbDA neurons. Subsequently, we deleted Wnt1 in the ventral Mb and show a continued late requirement for Wnt1 in MbDA neuron development, but not in LMX1a-expressing progenitors. Specifically, Wnt1 deletion disrupts the birthdating of MbDA neurons and causes a depletion of MbDA neurons positioned medially and a concomitant expansion of MbDA neurons positioned laterally during embryogenesis. Collectively, our analyses resolve the spatial and temporal function of Wnt1 in Mb and Cb patterning and in MbDA neuron development in vivo.