Lineage tracing axial progenitors using Nkx1.2CreERT2 mice defines their trunk and tail contributions

Background:

During mouse embryonic development, cells undergo a series of transformations. One of them is the formation of the paraxial mesoderm and neural tube, both thought to originate from the same cells, called Neuro-Mesodermal Progenitors (NMPs – for a more detailed introduction to NMPs you can refer to my previous highlight ¹). NMPs are transient and very rare in the embryo, complicating efforts to study their development. Recently, two reports deciphered the transcriptomic signature of NMPs in the embryo ^2,3. They identified these cells by the dual expression of Sox2 (a neural marker) and T (a mesoderm marker). From then, they identified unique transcription factors (TFs) expressed in these cells, such as Nkx1.2 Cdx2 and Cdx4 at E8.5, plus Tbx6 at E9.5. In their new preprint, Albors and colleagues extend this work, providing a comprehensive characterisation of the expression pattern of Nkx1.2 (a homeobox transcription factor), and identified Nkx1.2+ progeny cells by lineage tracking.

The data

First, in situ hybridization (ISH) was performed on Nkx1.2 in the mouse embryo from E6.0 to E12.5. Expression is first detected in E7.5 embryos in the posterior epiblast near where NMPs originate and is later seen in the neural tube and surrounding mesenchyme. Its expression also colocalised with Sox2+ and T+ regions. Following the characterisation of its expression pattern, they tagged Nkx1.2 with a CreERT2 cassette, then crossed it with R26R-EYFP (loxP flanked YFP) in order to permanently label progenies of all Nkx2.1+ cells. Tamoxifen was added at E7.5 and progeny cells were tracked at several stages. Nkx1.2+ cells at E7.5 are specialising into neurectoderm, several mesoderm tissues (excluding notochord) and, surprisingly, a few hindgut cells. These progenitors were maintained until E10.5 but no longer contributed to hindgut and surface ectoderm, rather labelling mostly trunk and tail tissues originating from the tail bud.

Why did I choose this article?

I was particularly interested by the lineage tracing experiments: using CreERT2 is a great way to understand TF dynamics as one can control the time of labelling. The authors have also produced compelling imaging data. They provided a thorough characterisation of a new marker that will allow the scientific community to perform deeper studies on NMPs.

Image reproduced from Albors et al., 2018 Fig 5B and 5Bd (section d)

Open questions

• I wonder if the authors plan to give Tamoxifen at earlierstages: this could potentially identify younger progenitor cells? ISH might not be sensitive enough to detect low transcript levels.
• Adding to that comment, Nkx1.2+ progeny cells were found in the hindgut (endoderm). So, it indicates that Nkx1.2 is expressed in a population that give rise to the three germ layers. Do the authors think that Nkx1.2 is actually labelling a pluripotent progenitor population?
• Is Nkx1.2 expressed in in vitro-derived NMPs? Transcriptomic comparisons between in vitro and in vivo populations would be something to look forward to, as well as ChIP-sequencing to unveil its targets.

Related research:

1. Edri, S., Hayward, P., Baillie-johnson, P. & Steventon, B. An Epiblast Stem Cell derived multipotent progenitor population for axial extension. (2018). doi:10.1101/242461
2. Gouti, M. et al. A Gene Regulatory Network Balances Neural and Mesoderm Specification during Vertebrate Trunk Development. Dev Cell 41, 243–261.e7 (2017).
3. Koch, F. et al. Antagonistic Activities of Sox2 and Brachyury Control the Fate Choice of Neuro-Mesodermal Progenitors. Dev Cell 42, 514–526.e7 (2017).

Tags: lineage tracing, mouse, nkx1.2, nmp

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