Stem cells that indirectly generate differentiated cells through intermediate progenitors drives vertebrate brain evolution. Due to a lack of lineage information, how stem cell functionality, including the competency to generate intermediate progenitors, becomes extinguished during progenitor commitment remains unclear. Type II neuroblasts in fly larval brains divide asymmetrically to generate a neuroblast and a progeny that commits to an intermediate progenitor (INP) identity. We identified Tailless (Tll) as a master regulator of type II neuroblast functional identity, including the competency to generate INPs. Successive expression of transcriptional repressors functions through Hdac3 to silence tll during INP commitment. Reducing repressor activity allows re-activation of Notch in INPs to ectopically induce tll expression driving supernumerary neuroblast formation. Knocking down hdac3 function prevents downregulation of tll during INP commitment. We propose that continual inactivation of stem cell identity genes allows intermediate progenitors to stably commit to generating diverse differentiated cells during indirect neurogenesis.
Sequencing data have been deposited in GEO under accession codes GSE152636.
Sequential activation of transcriptional repressors promotes progenitor commitment by silencing stem cell identity genesNCBI Gene Expression Omnibus, GSE152636.
- Hideyuki Komori
- Cheng-Yu Lee
- Hideyuki Komori
- Cheng-Yu Lee
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
- Claude Desplan, New York University, United States
© 2020, Rives- Quinto et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
The expression of Fibroblast growth factors (Fgf) ligands in a specialized epithelial compartment, the Apical Ectodermal Ridge (AER), is a conserved feature of limb development across vertebrate species. In vertebrates, Fgf 4, 8, 9, and 17 are all expressed in the AER. An exception to this paradigm is the salamander (axolotl) developing and regenerating limb, where key Fgf ligands are expressed in the mesenchyme. The mesenchymal expression of Amex.Fgf8 in axolotl has been suggested to be critical for regeneration. To date, there is little knowledge regarding what controls Amex.Fgf8 expression in the axolotl limb mesenchyme. A large body of mouse and chick studies have defined a set of transcription factors and canonical Wnt signaling as the main regulators of epidermal Fgf8 expression in these organisms. In this study, we address the hypothesis that alterations to one or more of these components during evolution has resulted in mesenchymal Amex.Fgf8 expression in the axolotl. To sensitively quantify gene expression with spatial precision, we combined optical clearing of whole-mount axolotl limb tissue with single molecule fluorescen in situ hybridization and a semi-automated quantification pipeline. Several candidate upstream components were found expressed in the axolotl ectoderm, indicating that they are not direct regulators of Amex.Fgf8 expression. We found that Amex.Wnt3a is expressed in axolotl limb epidermis, similarly to chicken and mouse. However, unlike in amniotes, Wnt target genes are activated preferentially in limb mesenchyme rather than in epidermis. Inhibition and activation of Wnt signaling results in downregulation and upregulation of mesenchymal Amex.Fgf8 expression respectively. These results implicate a shift in tissue responsiveness to canonical Wnt signaling from epidermis to mesenchyme as one step contributing to the unique mesenchymal Amex.Fgf8 expression seen in the axolotl.
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