Notch Signal Transduction in Vertebrate Neurogenesis
Notch Pathway Overview
Notch signaling is regulated by cell-cell interactions, with Notch receptors (of which there are four in mammals, Notch1-4) on one cell activated by ligands, the Delta-like (Dll1,3,4) and Jagged (Jag1,2) proteins, expressed on neighboring cells. Receptor stimulation involves dynamin-mediated endocytosis on the signal-sending and signal-receiving cells, with ubiquitination of the ligands (by the E3 ligase Mindbomb1 [Mib1], for example) and receptors (by the E3 ligase Deltex [Dx], for example) employed to drive internalization. Upon receptor activation, the intracellular domain of Notch (NICD) is ultimately cleaved at site 3 (S3) by the Presenilin proteases (Psen1/2) of the g-secretase complex and translocates to the nucleus to associate with CBF1 (also called RBP-J or CSL) and Mastermind-like (Maml) proteins to activate transcription of target genes.
Fig.1 Schematic of the Core Elements of the Notch Signaling Pathway. (Pierfelice, 2011)
Vertebrate Neurogenesis
During the initial stages of development, the vertebrate embryo undergoes a dorsal invagination of the neuroectoderm to form the neural tube. This structure, which subsequently will generate the brain and spinal cord as well as the neural crest derivatives, is initially a monostratified epithelium with its apical side forming the lumenal surface. As development proceeds neural precursors divide vigorously in an unsynchronized manner, cellular density increasing dramatically and acquiring a highly packed, pseudostratified disposition characterized by the presence of their nuclei at different levels depending on the cell cycle stage they are. A hallmark of the neural precursors is therefore the to-and-fro displacement of the nucleus during the cell cycle, a process that is referred to as interkinetic nuclear migration (INM). This nuclear movement spans the entire apical-basal axis of the cell, with the nucleus migrating to the basal side during the first gap (G1) phase of the cell cycle, staying at the basal side during the DNA synthesis phase (S-phase), migrating back to the apical side during the second gap (G2) phase, and undergoing mitosis (M) at the apical side.
Fig.2 Notch in Adult SVZ Neurogenesis. (Pierfelice, 2011)
Notch Signal Transduction in Vertebrate Neurogenesis
The hypothesis that Notch activation in vertebrates would inhibit neuronal differentiation was derived from classic fly genetic studies, which found that disruption of the Notch pathway led to excessive neuronal differentiation. Those studies, together with the identification of lateral inhibition during neurogenesis in grasshopper embryos, and vulval development in nematodes, led to early work in mammalian cell lines and Xenopus and chick embryos showing that Notch activation in vertebrate cells influenced cell fate and inhibited neuronal differentiation. Indeed, recent work in the mouse brain has continued to support the model that lateral inhibition regulates the balance between neural progenitor maintenance and neuronal differentiation. The realization that Notch signaling performed a similar function during both fly and vertebrate neural development led to the identification of many vertebrate orthologs of fly pathway components that, for the most part, exhibited functions predicted by their roles in flies. As a result, for several years, the field was dominated by studies drawing parallels between Notch function in flies and vertebrates.
Notch signaling also plays a primary role in the control of neurogenesis in the Subgranular Zone (SGZ) of the Dentate Gyrus (DG). Notch receptors are expressed throughout the DG including on the NSCs and progenitors in the SGZ. Active Notch signaling is prominent in both radial and horizontal NSCs (Type-1 cells) but is absent from the IPs (Type-2 cells) and immature neurons (Type-3 cell). The transcription of the Notch target Hes5 efficiently discriminates the NSCs from other cells including proliferative committed progenitors in the DG. The Notch ligand JAGGED1 is preferentially expressed by IPCs and neurons in the DG, although its expression has also been found in radial glia-like stem cells. Hes5 is a target of Notch signaling in the central nervous system and a good indicator of Notch activity. Using reporter mice where GFP is driven by the regulatory elements of the Hes5 gene (Hes5::GFP), both radial and horizontal Type-1 cells were found to express Hes5 in the adult DG.
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Reference
- Pierfelice, T., et al. Notch in the vertebrate nervous system: an old dog with new tricks. Neuron. 2011, 69(5): 840-55.
