Adult neural stem cells (NSCs) derive from embryonic precursors, but little is known about how or when this occurs. We have addressed this issue using single-cell RNA sequencing at multiple developmental time points to analyze the embryonic murine cortex, one source of adult forebrain NSCs. We computationally identify all major cortical cell types, including the embryonic radial precursors (RPs) that generate adult NSCs. We define the initial emergence of RPs from neuroepithelial stem cells at E11.5. We show that, by E13.5, RPs express a transcriptional identity that is maintained and reinforced throughout their transition to a non-proliferative state between E15.5 and E17.5. These slowly proliferating late embryonic RPs share a core transcriptional phenotype with quiescent adult forebrain NSCs. Together, these findings support a model wherein cortical RPs maintain a core transcriptional identity from embryogenesis through to adulthood and wherein the transition to a quiescent adult NSC occurs during late neurogenesis.

The mechanisms instructing genesis of neuronal subtypes from mammalian neural precursors are not well understood. To address this issue, we have characterized the transcriptional landscape of radial glial precursors (RPs) in the embryonic murine cortex. We show that individual RPs express mRNA, but not protein, for transcriptional specifiers of both deep and superficial layer cortical neurons. Some of these mRNAs, including the superficial versus deep layer neuron transcriptional regulators Brn1 and Tle4, are translationally repressed by their association with the RNA-binding protein Pumilio2 (Pum2) and the 4E-T protein. Disruption of these repressive complexes in RPs mid-neurogenesis by knocking down 4E-T or Pum2 causes aberrant co-expression of deep layer neuron specification proteins in newborn superficial layer neurons. Thus, cortical RPs are transcriptionally primed to generate diverse types of neurons, and a Pum2/4E-T complex represses translation of some of these neuronal identity mRNAs to ensure appropriate temporal specification of daughter neurons.

Highlights d Pdgfra-expressing mesenchymal cells from uninjured digits establish the blastema d Adult digit tip regeneration is distinct from embryonic digit development d The blastema state is environmentally determined d The regenerative environment enables mesenchymal lineage plasticity

Highlights d Cortex and GE-derived V-SVZ NSCs share a common forebrain NSC signature d Embryonic NSCs transition to dormant adult NSCs in the first postnatal week d Adult dormant NSCs acquire a development-like state when reactivated d Cortex-derived adult NSCs acquire a GE-like state when activated to make neurons

The neural stem cell decision to self-renew or differentiate is tightly regulated by its microenvironment. Here, we have asked about this microenvironment, focusing on growth factors in the embryonic cortex at a time when it is largely comprised of neural precursor cells (NPCs) and newborn neurons. We show that cortical NPCs secrete factors that promote their maintenance, while cortical neurons secrete factors that promote differentiation. To define factors important for these activities, we used transcriptome profiling to identify ligands produced by NPCs and neurons, cell-surface mass spectrometry to identify receptors on these cells, and computational modeling to integrate these data. The resultant model predicts a complex growth factor environment with multiple autocrine and paracrine interactions. We tested this communication model, focusing on neurogenesis, and identified IFNγ, Neurturin (Nrtn), and glial-derived neurotrophic factor (GDNF) as ligands with unexpected roles in promoting neurogenic differentiation of NPCs in vivo.

A Shared Transcriptional Identity for Forebrain and Dentate Gyrus Neural Stem Cells from Embryogenesis to Adulthood Abbreviated Title (max 50 characters)Shared identity between V-SVZ and SGZ NSCs

Long-term cognitive impairments are common in pediatric brain cancer survivors. While these impairments are thought to arise following radiation treatment, recent reports suggest a link to tumor-specific mechanisms. We therefore hypothesized that pediatric brain tumors, more specifically medulloblastoma (MB), can directly affect neural stem and precursor cell function in the forebrain stem cell niche—the subventricular zone (V-SVZ) —by secreting bioactive factors. Mice harboring subcutaneous flank MB tumors had fewer proliferating neural precursor cells in the V-SVZ than controls as well as decreased olfactory bulb neurogenesis and white matter oligodendrogenesis. To assess the effects of the MB secretome in the brain, concentrated conditioned media from MB cell lines (MB-CM) was injected intracerebroventricular (ICV) into mouse pups. ICV injection of MB-CM decreased neural precursor cell proliferation as well as decreased numbers of V-SVZ neurospheres in culture. MB-CM from multiple cell lines decreased V-SVZ neurosphere number and promoted astrocyte differentiation in culture. To identify the ligands contributing to the phenotype, an interaction model was developed extracting ligands from MB microarray data, networking them to receptors on NSCs. Of the predicted ligands, IL6-family cytokine expression and secretion was validated in MB cells. When added in culture, recombinant IL6, IL11, and CT1 decreased neurosphere number and ICV injection of IL11 into mouse pups decreased V-SVZ neural precursor cell proliferation. Overall, this work demonstrates that medulloblastoma secretes bioactive compounds that perturb neural stem cell function and the circuity involved in normal cognitive function. This abstract is also being presented as Poster B64. Citation Format: Alexander Gont, Jaclin V. Simonetta, Jenna Park, Alice R. Shan, Michael J. Borrett, Michael D. Taylor, Freda D. Miller, David R. Kaplan. Acting at a distance: Medulloblastoma-secreted ligands disrupt normal neural stem cell function [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr PR15.