Excessive Wnt/beta-catenin signaling promotes midbrain floor plate neurogenesis, but results in vacillating dopamine progenitors

Mol Cell Neurosci. 2015 Sep;68:131-42. doi: 10.1016/j.mcn.2015.07.002. Epub 2015 Jul 9.

Abstract

The floor plate (FP), a ventral midline structure of the developing neural tube, has differential neurogenic capabilities along the anterior-posterior axis. The midbrain FP, unlike the hindbrain and spinal cord floor plate, is highly neurogenic and produces midbrain dopaminergic (mDA) neurons. Canonical Wnt/beta-catenin signaling, at least in part, is thought to account for the difference in neurogenic capability. Removal of beta-catenin results in mDA progenitor specification defects as well as a profound reduction of neurogenesis. To examine the effects of excessive Wnt/beta-catenin signaling on mDA specification and neurogenesis, we have analyzed a model wherein beta-catenin is conditionally stabilized in the Shh+domain. Here, we show that the Foxa2+/Lmx1a+ domain is extended rostrally in mutant embryos, suggesting that canonical Wnt/beta-catenin signaling can drive FP expansion along the rostrocaudal axis. Although excess canonical Wnt/beta-catenin signaling generally promotes neurogenesis at midbrain levels, less tyrosine hydroxylase (Th)+, mDA neurons are generated, particularly impacting the Substantia Nigra pars compacta. This is likely because of improper progenitor specification. Excess canonical Wnt/beta-catenin signaling causes downregulation of net Lmx1b, Shh and Foxa2 levels in mDA progenitors. Moreover, these progenitors assume a mixed identity to that of Lmx1a+/Lmx1b+/Nkx6-1+/Neurog1+ progenitors. We also show by lineage tracing analysis that normally, Neurog1+ progenitors predominantly give rise to Pou4f1+ neurons, but not Th+ neurons. Accordingly, in the mutant embryos, Neurog1+ progenitors at the midline generate ectopic Pou4f1+ neurons at the expense of Th+ mDA neurons. Our study suggests that an optimal dose of Wnt/beta-catenin signaling is critical for proper establishment of the mDA progenitor character. Our findings will impact embryonic stem cell protocols that utilize Wnt pathway reagents to derive mDA neuron models and therapeutics for Parkinson's disease.

Keywords: Floor plate; Lmx1b; Midbrain dopaminergic neurons; Wnt/beta-catenin.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Age Factors
  • Animals
  • Basic Helix-Loop-Helix Transcription Factors / genetics
  • Basic Helix-Loop-Helix Transcription Factors / metabolism
  • Body Patterning / genetics
  • Dopamine / metabolism*
  • Embryo, Mammalian
  • Embryonic Stem Cells / physiology*
  • Female
  • Gene Expression Regulation, Developmental / genetics*
  • Hedgehog Proteins / genetics
  • Hedgehog Proteins / metabolism
  • Male
  • Mesencephalon / cytology*
  • Mesencephalon / embryology
  • Mice
  • Mice, Transgenic
  • Nerve Tissue Proteins / genetics
  • Nerve Tissue Proteins / metabolism
  • Neurogenesis / genetics*
  • RNA, Untranslated / genetics
  • RNA, Untranslated / metabolism
  • Tyrosine 3-Monooxygenase / metabolism
  • Wnt Signaling Pathway / genetics
  • Wnt Signaling Pathway / physiology*
  • beta Catenin / genetics
  • beta Catenin / metabolism*

Substances

  • Basic Helix-Loop-Helix Transcription Factors
  • CTNNB1 protein, mouse
  • Gt(ROSA)26Sor non-coding RNA, mouse
  • Hedgehog Proteins
  • Nerve Tissue Proteins
  • RNA, Untranslated
  • Shh protein, mouse
  • beta Catenin
  • Neurog1 protein, mouse
  • Tyrosine 3-Monooxygenase
  • Dopamine