Neogenin and RGMa Control Neural Tube Closure and Neuroepithelial Morphology by Regulating Cell Polarity

Abstract

In humans, neural tube closure defects occur in 1:1000 pregnancies. The design of new strategies for the prevention of such common defects would benefit from an improved understanding of the molecular events underlying neurulation. Neural fold elevation is a key morphological process that acts during neurulation to drive neural tube closure. However, to date, the molecular pathways underpinning neural fold elevation have not been elucidated. Here, we use morpholino knock-down technology to demonstrate that Repulsive Guidance Molecule (RGMa)-Neogenin interactions are essential for effective neural fold elevation during Xenopus neurulation and that loss of these molecules results in disrupted neural tube closure. We demonstrate that Neogenin and RGMa are required for establishing the morphology of deep layer cells in the neural plate throughout neurulation. We also show that loss of Neogenin severely disrupts the microtubule network within the deep layer cells suggesting that Neogenin-dependent microtubule organization within the deep cells is essential for radial intercalation with the overlying superficial cell layer, thereby driving neural fold elevation. In addition, we show that sustained Neogenin activity is also necessary for the establishment of the apicobasally polarized pseudostratified neuroepithelium of the neural tube. Therefore, our study identifies a novel signaling pathway essential for radial intercalation and epithelialization during neural fold elevation and neural tube morphogenesis.

Neogenin Regulates Skeletal Myofiber Size and Focal Adhesion Kinase and Extracellular Signal-regulated Kinase Activities In Vivo and In Vitro

“…Thus, although the phenotype observed in Neo1 Gt/Gt mice may be similar to that achieved with a true null allele, a final conclusion on the full role of neogenin in myogenesis must await the generation of a targeted, null mutation. Morpholino-mediated knockdown of neogenin in zebrafish and Xenopus embryos resulted in defective neural tube development, including failure of cavitation or closure, respectively (Mawdsley et al, 2004;Kee et al, 2008). These phenotypes were not seen in Neo1 Gt/Gt mouse embryos (Hong and Krauss, unpublished data).…”

“…RGMc, but not RGMa or RGMb, is expressed in developing muscle and is induced during myoblast differentiation in vitro (Kuninger et al, 2004(Kuninger et al, , 2006Niederkofler et al, 2004;Oldekamp et al, 2004). Although RGMa has been demonstrated to be a ligand for neogenin in the developing CNS (Matsunaga et al, 2004(Matsunaga et al, , 2006Rajagopalan et al, 2004;Conrad et al, 2007;Kee et al, 2008), it is not clear that RGMc has a similar function in muscle. We therefore assessed whether soluble netrin-2 and/or RGMc were capable of inducing FAK and ERK phosphorylation in myoblasts and whether neogenin was required for such activity through analysis of Neo1 ϩ/ϩ and Neo1 Gt/Gt myoblasts.…”

“…Neogenin functions as a receptor for RGMa in axonal chemorepulsion and neuronal survival and differentiation (Matsunaga et al, 2004;Rajagopalan et al, 2004;Matsunaga et al, 2006;Conrad et al, 2007). Furthermore, RGMa-neogenin signaling is required for neural tube closure in Xenopus embryos (Kee et al, 2008). RGMc (also known as hemojuvelin [Hjv] or Hfe2) is expressed mainly in liver and skeletal muscle and is involved in iron metabolism (Kuninger et al, 2004;Niederkofler et al, 2004;Oldekamp et al, 2004;Papanikolaou et al, 2004).…”

RGMs: Structural Insights, Molecular Regulation, and Downstream Signaling

“…These data show that RGMs are a structural bridge between Neogenin and BMP signaling, and inform that a clustering mechanism may be important in the activation of these signaling pathways. [29,30], early stages of neurulation [31], CD4+ T cell adhesion and activation [32], and leukocyte migration [33]. This can be either contact dependent (adhesive) or mediated by gradients established by cleaved extracellular RGM isoforms.…”

“…Epithelial sheets undergo choreographed movements to generate complex structures, such as the neural tube. Early depletion of Neogenin or RGMa in the neuroepithelium leads to loss of adhesion and apicobasal polarity and, as a result, a failure in neural tube closure [31,47,60,61]. Ecadherin-mediated cell-cell adhesion found at adherens junctions (AJs) has a key role in maintaining the fidelity of the epithelium.…”