Dynamic Interactions between Intermediate Neurogenic Progenitors and Radial Glia in Embryonic Mouse Neocortex: Potential Role in Dll1-Notch Signaling

Abstract

The mammalian neocortical progenitor cell niche is composed of a diverse repertoire of neuroepithelial cells, radial glia (RG), and intermediate neurogenic progenitors (INPs). Previously, live-cell imaging experiments have proved crucial in identifying these distinct progenitor populations, especially INPs, which amplify neural output by undergoing additional rounds of proliferation before differentiating into new neurons. INPs also provide feedback to the RG pool by serving as a source of Delta-like 1 (Dll1), a key ligand for activating Notch signaling in neighboring cells, a well-known mechanism for maintaining RG identity. While much is known about Dll1-Notch signaling at the molecular level, little is known about how this cell-cell contact dependent feedback is transmitted at the cellular level. To investigate how RG and INPs might interact to convey Notch signals, we used high-resolution live-cell multiphoton microscopy (MPM) to directly observe cellular interactions and dynamics, in conjunction with Notch-pathway specific reporters in the neocortical neural stem cell niche in organotypic brain slices from embryonic mice. We found that INPs and RG interact via dynamic and transient elongate processes, some apparently long-range (extending from the subventricular zone to the ventricular zone), and some short-range (filopodia-like). Gene expression profiling of RG and INPs revealed further progenitor cell diversification, including different subpopulations of Hes1+ and/or Hes5+ RG, and Dll1+ and/or Dll3+ INPs. Thus, the embryonic progenitor niche includes a network of dynamic cell-cell interactions, utilizing different combinations of Notch signaling molecules to maintain and likely diversify progenitor pools.

Cadherin-based adhesions in the apical endfoot are required for active Notch signaling to control neurogenesis in vertebrates

“…The apical contact of neurons has been described previously, albeit at slightly later stages with a much lower incidence (Minaki et al, 2005;Ochiai et al, 2007). It has been reported that INPs also transiently retain the apical endfeet (Borrell et al, 2013;Nelson et al, 2013;Noctor et al, 2008 In the fly mechanosensory organ development, Delta-expressing cells extend actin-based filopodia to activate Notch signaling in distant cells by 'touch ' (de Joussineau et al, 2003). Together with the present findings, these cellular 'skills' that use cellular protrusions would make the actions of Notch signaling, which by nature only mediates a juxtamembrane communication, more versatile in diverse biological contexts.…”

N-cadherin-based adherens junction regulates the maintenance, proliferation, and differentiation of neural progenitor cells during development

“…42 After asymmetric neurogenic division in the developing mouse cortex, Delta-Notch signaling components are divided asymmetrically into daughter cells, and therefore Notch signaling in the nascent neuron or IP is lower than in the cell, which maintains the identity of NPCs. [44][45][46] These facts suggest the downregulation of Notch signaling may be important for the down-regulation of N-cadherin expression.…”

“…127 The nascent neurons and IPs inherited lower level of Delta-Notch signaling components than the cell, which maintains the identity of NPC. 44,46 The down-regulation of Notch signaling is important for the downregulation of N-cadherin and subsequent apical process detachment. 127 As described above, some studies show a functional link between Notch signaling and cadherin-mediated AJ.…”

Evolution and Development of the Mammalian Cerebral Cortex

“…Expression of Scrt2 suggested that IPs move from VZ to SVZ by a type of epithelial-mesenchymal transition (Itoh et al, 2013). Molecular analyses implicated IPs in Delta-Notch signaling (Kawaguchi et al, 2008; Nelson et al, 2013). Molecular comparisons are also telling us about the evolution of IPs (Lui et al, 2011; Molnár and Clowry, 2012).…”

“…IPs play an essential role in this process (Yoon et al, 2008). Specifically, IPs highly express Notch receptor ligands Delta-like 1 (Dll1) on both apical and basal IPs, and Dll3 on basal IPs only (Kawaguchi et al, 2008; Nelson et al, 2013). Interestingly, Dll1 protein is highly expressed on IP processes, including long apical processes that extend from the SVZ to the VZ where they contact mitotic RG at the apical surface (Nelson et al, 2013).…”