Understanding how heterogenous subtypes of neurons in the brain are established during embryogenesis is a vital endeavor in modern neuroscience. In terms of development, a critically important population of neurons to study are the midbrain dopamine neurons, which are clinically relevant due to their strong links to Parkinson’s Disease and schizophrenia, and functionally relevant due to their roles in cognition, reward behavior, and motor control. Midbrain dopamine neurons are comprised of several subtypes based on divergent circuits, physiology, and molecular marker expression, and are classically divided into three broad cell groups: the bilateral substantia nigra pars compacta, the medial ventral tegmental area, and the retrorubral field. The depletion of distinct cell groups of midbrain dopamine neurons is strongly linked to specific neurological disorders such as Parkinson’s Disease, where midbrain dopamine neurons are predominantly depleted in the substantia nigra pars compacta, and schizophrenia where midbrain dopamine neurons are predominantly depleted or dysfunctional in the medial ventral tegmental area. Understanding how different subpopulations of midbrain dopamine neurons are established in development and whether genetic lineage is directly related to specific subpopulations of midbrain dopamine neurons may be instructive in cell-based therapeutic approaches to rescue the loss of specific midbrain dopamine neuron subtypes. However, it is unclear how the diversity of midbrain dopamine subtypes is established during development. Midbrain dopamine neurons are derived from the ventral mesencephalon and it has been previously shown that Wnt1-expressing progenitors in the v.Mes contribute to midbrain dopamine neurons in the adult brain. We tested the hypothesis that Wnt1-expressing progenitors in the ventral mesencephalon give rise to different subtypes of midbrain dopamine neurons at distinct developmental timepoints. In Chapter 2, I demonstrated, with genetic-inducible fate mapping, that the Wnt1 lineage is the in vivo source of progenitors that gives rise to midbrain dopamine neurons positioned in the substantia nigra pars compacta, the medial ventral tegmental area, and the retrorubral field over a prolonged temporal window. Interestingly, Wnt1-expressing progenitors give rise to midbrain dopamine neurons over this prolonged time period (E7.5-E13.5) with two peaks at E9.5 and E11.5. Also, our fate-mapping results revealed that biochemically defined MbDA neurons: calbindin-expressing, GIRK2-expressing, and calretinin-expressing dopamine neuron subpopulations are derived from the Wnt1 lineage when marked across these developmental points with two peaks of contribution. Collectively, these findings led us to further characterize the molecular identity and spatial distribution of Wnt1-expressing midbrain dopamine progenitors from E8.5-E12.5 (when we marked cells using GIFM). Our findings showed that the Wnt1 expression domain dynamically changed during the marking period and that Wnt1-expressing progenitors had distinct molecular identities that correlated with specific spatial locations in the ventral mesencephalon. Collectively, my findings suggested that Wnt1 expression in midbrain dopamine progenitors is necessary from E7.5 to E13.5 to give rise to midbrain dopamine neurons in the adult brain. Therefore, in Chapter 3, I utilized a conditional Wnt1 allele (Wnt1fl) generated in our lab and used En1Cre and ShhCre mice to specifically delete Wnt1 in midbrain dopamine progenitors at distinct time points. The early temporal deletion of Wnt1 before E9.0 (En1Cre;Wnt1∆MHB/∆MHB conditional mutants) revealed that Wnt1 regulates midbrain patterning, the activation of Lmx1a expression, and the induction of all midbrain dopamine neurons at this early stage. The later temporal deletion of Wnt1 with ShhCre;Wnt1∆MHB/∆MHB conditional mutants revealed that Wnt1 has a continued role in a subset of midbrain dopamine neurons, plays a role in midbrain dopamine neuron differentiation, and controls cell cycle regulation. When considered together, our findings shed light on the critical role of Wnt1 in midbrain dopamine neuron development, and may have clinical applications in the design of induced pluripotent stem cells (iPSCs) for cell replacement therapies in midbrain dopamine neuron diseases such as Parkinson’s Disease. It also useful to generate specific cell types to study cellular and molecular mechanisms of disease.
Leder Brown, Ashly Christine,
"The Dynamic Role of Wnt1 in Midbrain Dopamine Neuron Development"
Neuroscience Theses and Dissertations.
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