ABSTRACT
How the sequential specification of neurons and progressive loss of potency associated with aging neural progenitors are regulated in vertebrate brain development is poorly understood. By examining a temporal differentiation lineage in the hindbrain, we here identify Tgfß as a switch signal that executes the transition between early and late phases of neurogenesis and concurrently constrains progenitor potency. Young progenitors have inherent competence to produce late-born neurons, but implementation of late-differentiation programs requires suppression of early identity genes achieved through temporally programmed activation of Tgfß downstream of Shh signaling. Unexpectedly, we find that sequentially occurring fate-switch decisions are temporally coupled, and onset of Tgfß signaling appears thereby to impact on the overall lifespan of the temporal lineage. Our study establishes Tgfß as a regulator of temporal identity and potency of neural stem cells, and provides proof of concept that Tgfß can be applied to modulate temporal specification of neurons in stem cell engineering.
Subject(s)
Central Nervous System/cytology , Gene Expression Regulation, Developmental/physiology , Neural Stem Cells/physiology , Neurogenesis/physiology , Protein Serine-Threonine Kinases/metabolism , Receptors, Transforming Growth Factor beta/metabolism , Signal Transduction/physiology , Age Factors , Animals , Cell Differentiation/genetics , Cell Differentiation/physiology , Cell Movement/genetics , Cells, Cultured , Chick Embryo , Embryo, Mammalian , Female , Gene Expression Regulation, Developmental/genetics , Homeobox Protein Nkx-2.2 , Homeodomain Proteins/genetics , Mice , Mice, Transgenic , Neural Tube , Organ Culture Techniques , Pregnancy , Protein Serine-Threonine Kinases/genetics , Receptor, Transforming Growth Factor-beta Type I , Receptors, Transforming Growth Factor beta/genetics , Signal Transduction/genetics , Transcription Factors/genetics , Zebrafish ProteinsABSTRACT
Dimerization is recognized as a crucial step in the activation of many plasma membrane receptors. However, a growing number of receptors pre-exist as dimers in the absence of ligand, indicating that, although necessary, dimerization is not always sufficient for signaling. The p75 neurotrophin receptor (p75(NTR)) forms disulfide-linked dimers at the cell surface independently of ligand binding through Cys257 in its transmembrane domain. Here, we show that crosslinking of p75(NTR) dimers by cysteine-scanning mutagenesis results in constitutive, ligand-independent activity in several pathways that are normally engaged upon neurotrophin stimulation of native receptors. The activity profiles of different disulfide-crosslinked p75(NTR) mutants were similar but not identical, suggesting that different configurations of p75(NTR) dimers might be endowed with different functions. Interestingly, crosslinked p75(NTR) mutants did not mimic the effects of the myelin inhibitors Nogo or MAG, suggesting the existence of ligand-specific activation mechanisms. Together, these results support a conformational model of p75(NTR) activation by neurotrophins, and reveal a genetic approach to generate gain-of-function receptor variants with distinct functional profiles.
