Abstract
Cerebral cortex projection neurons (PNs) are generated from intermediate progenitors (IPs), which are in turn derived from radial glial progenitors (RGPs). To investigate developmental processes in IPs, we profiled IP transcriptomes in embryonic mouse neocortex, using transgenic Tbr2-GFP mice, cell sorting, and microarrays. These data were used in combination with in situ hybridization to ascertain gene sets specific for IPs, RGPs, PNs, interneurons, and other neural and non-neural cell types. RGP-selective transcripts (n = 419) included molecules for Notch receptor signaling, proliferation, neural stem cell identity, apical junctions, necroptosis, hippo pathway, and NF-κB pathway. RGPs also expressed specific genes for critical interactions with meningeal and vascular cells. In contrast, IP-selective genes (n = 136) encoded molecules for activated Delta ligand presentation, epithelial-mesenchymal transition, core planar cell polarity (PCP), axon genesis, and intrinsic excitability. Interestingly, IPs expressed several “dependence receptors” (Unc5d, Dcc, Ntrk3, and Epha4) that induce apoptosis in the absence of ligand, suggesting a competitive mechanism for IPs and new PNs to detect key environmental cues or die. Overall, our results imply a novel role for IPs in the patterning of neuronal polarization, axon differentiation, and intrinsic excitability prior to mitosis. Significantly, IPs highly express Wnt-PCP, netrin, and semaphorin pathway molecules known to regulate axon polarization in other systems. In sum, IPs not only amplify neurogenesis quantitatively, but also molecularly “prime” new PNs for axogenesis, guidance, and excitability.
Highlights
Intermediate progenitors (IPs) are a type of cortical progenitors “intermediate” in the lineage from radial glial progenitors (RGPs), which produce IPs, and projection neurons (PNs), which are generated from IPs (Haubensak et al, 2004; Miyata et al, 2004; Noctor et al, 2004)
Despite sorting by DNA content, postmitotic cells were not excluded, as they were well represented in the transcriptome results; the most likely explanation is that cell dissociation was incomplete, with doublets sorted as high-DNA cells
Gene enrichment was expressed quantitatively as log2 of the fold change, which was positive for genes enriched in GFP+ cells, and negative for genes enriched in GFP− cells
Summary
Intermediate progenitors (IPs) are a type of cortical progenitors “intermediate” in the lineage from radial glial progenitors (RGPs), which produce IPs, and projection neurons (PNs), which are generated from IPs (Haubensak et al, 2004; Miyata et al, 2004; Noctor et al, 2004). RGPs have high self-renewal capacity and multilineage differentiation potential, and are considered to be a class of neural stem cells (NSCs) (Taverna et al, 2014). IPs have low proliferative capacity and single lineage commitment to produce only glutamatergic PNs, and are neural progenitor cells. IPs generate the vast majority (possibly all) of the PNs in all cortical layers, including CajalRetzius cells and subplate neurons (Haubensak et al, 2004; Kowalczyk et al, 2009; Mihalas et al, 2016). IPs are thought to serve as a cellular substrate for development of gyral folds (Kriegstein et al, 2006; Hevner, 2016; Toda et al, 2016)
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