Molecular mechanisms of NMC plasticity and fate commitment

Abstract

Elongation of the anteroposterior (AP) axis in vertebrates relies on a pool of stem cell-like Neuromesodermal competent cells (NMCs) that supply the neural tube and paraxial mesoderm with progenitors during embryogenesis. A finely tuned balance between NMC self-renewal and commitment to neural or mesodermal fates governs NMC homeostasis. This balance is orchestrated by converging Wnt, FGF and retinoic acid (RA) signals that assemble a gene-regulatory network (GRN), anchored by an autoregulatory loop between Wnt3a and Tbxt which is essential in maintaining NMC self-renewal and mesoderm lineage competency. Disrupting genes in the NMC-GRN is known to collapse a common network, evidenced by a shared loss of progenitors and a truncation of the AP axis. Therefore, decoding NMC biology demands mapping the transcriptional and epigenetic interactions that wire the network that sustains NMC homeostasis during axis elongation.
Members of the Specificity protein (Sp) family of zinc finger transcription factors (TF), particularly Sp5 and Sp8, are candidate regulators of the NMC-GRN. Their combined loss phenocopies the Wnt3a null phenotype, implicating them in the Wnt3a-Tbxt autoregulatory loop and, by extension, the maintenance of NMC self-renewal and mesoderm formation. Yet, details regarding the mechanisms by which Sp proteins interface with the wider NMC-GRN remains unclear.
In this thesis, we illustrate that Sp proteins are critical regulators of NMC homeostasis throughout axis elongation. We find that Sp5 and Sp8, together with other NMC-GRN regulators, control the network through reciprocal interactions and converge on a cis-regulatory element (cRE) downstream of Wnt3a. In gastruloid models, we show this element is essential for Wnt3a expression, and its deletion collapses the Wnt3a-Tbxt autoregulatory loop, driving premature depletion of axial progenitors. Using single-cell multi-omics, we show that Sp8 is required to sustain NMC homeostasis during later stages of axis elongation. Loss of Sp8 was shown to remodel the epigenetic landscape and induce a transcriptional response that destabilizes the NMC-GRN, biasing NMCs toward neural lineages at the expense of mesoderm and self-renewal. Importantly, we link this bias and the resulting exhaustion of the NMC pool to the absence of tail vertebrae and an expanded posterior neural plate, a feature we propose underlies the spina bifida–like phenotype in Sp8 mutants. Together, our findings establish Sp proteins as central components of the NMC-GRN and provide insight linking the fundamental molecular regulation of axial progenitors to the aetiology of clinically relevant phenotypes.

Thesis is embargoed until 31 July 2031.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	National Cancer Institute
Supervisor	Yaser Atlasi (Supervisor), Terry Yamaguchi (Supervisor) & Nick Orr (Supervisor)