Non-neural surface ectodermal rosette formation and F-actin dynamics drive mammalian neural tube closure

Loss of Grhl3 is correlated with altered cellular protrusions in the non‐neural ectoderm during neural tube closure

“…32,35,36 In particular, these studies implicated GRHL3 in several roles within the NNE with regards to NTC, including maintaining the proper balance between NE and NNE tissues just prior to neurulation, 43 directing acto-myosin accumulation towards the midline during cranial and spinal neurulation, 36 inducing a change in morphology of NNE cells necessary for shaping the zippering fork and regulating the cellular projections needed to accomplish fusion. 35 These previous studies have contributed important knowledge to an understanding of the role of Grhl3 in the NNE. Our contribution here is to extend this knowledge by focusing specifically on the time and place of GRHL3 requirement in spinal NTC.…”

“…In a recent study, loss of Grhl3 was indeed shown to be associated with loss of cellular projections emanating from the NNE. 35 However, this was examined at late stages after stalling and after eversion and disruption of the architecture of the neural folds. Our study is the first to use SEM to examine the effect that Grhl3 loss has on cellular projections both at the time of closure delay (E8.5) and at stalling of the zippering fork (E9.5), before eversion of the neural folds.…”

“…3 The functions of the NNE in zippering include not only directing the actin cytoskeleton towards the dorsal midline, 15,33 and generating the protrusions which initiate contact between opposing folds, 11,12,15 but also the formation of a rosette of cells in the NNE which give the zippering fork its distinctive shape. 34,35 A recent study identified that Grhl3 cre/cre mice display actin cytoskeletal defects in the NNE during spinal NTC. 36 During neurulation, Grhl3 mutants display less actomyosin localized to the junction of the NNE and neural folds This phenotype is driven by the downregulation of a guanine exchange factor which regulates the activity of small RhoGTPases like RHOA or CDC42, causing a reduction in activated RHOA and subsequent reduction in actomyosin localized to the dorsal midline at the boundary of the NE and NNE.…”

“…15 The final closure of the posterior neuropore involves discrete bundles of lamellipodia emanating from the NNE at the zippering fork and in regularly spaced cells ahead of the fork, with little to no filopodia observed. 15,35 Interestingly, conditional deletions of lamellipodia or filopodia in the NNE by knocking out Rac1 (lamellipodia) or Cdc42 (filopodia), result in NTDs biased along the R-C axis 15 in a fashion reminiscent of the Grhl3 and Grhl2 phenotypes. 21,23 The bias of Grhl3 mutants towards spina bifida and the similarity in phenotype to Rac1 mutants, which develop spina bifida at much higher proportions than exencephaly, combined with the known role of GRHL3 in directing the actin cytoskeleton during NTC in other contexts, 41 suggests that GRHL3 is acting upstream in the regulation of protrusive behaviors by the NNE.…”

Spotlight on the Granules (Grainyhead-Like Proteins) – From an Evolutionary Conserved Controller of Epithelial Trait to Pioneering the Chromatin Landscape

“…The remodeling of five or more polarized epithelial cells converging radially around a central point of fusion to form transient “rosette” like structures are identified during the formation of multiple organ systems, including surface ectodermal lineage specification ( Afonso and Henrique, 2006 ; Nishimura and Takeichi, 2008 ; Harding et al, 2014 ). Genetic fate mapping using Grhl3 Cre/+ mice have reported that the rosette forming cells of the surface ectoderm are Grhl3-expressing lineage cells, and Grhl3 mutants showed severe disruption of rosette formation, exhibiting fully penetrant spina bifida ( Molè et al, 2020 ; Zhou et al, 2020 ). Single-cell RNA sequencing of neural rosettes in vitro , generated from human induced pluripotent stem cells, showed GRHL2 and GRHL3 were highly expressed in early rosettes highlighting the role for GRHL TFs in neurulation in humans ( Shang et al, 2018 ).…”