The Drosophila Metastasis Suppressor Gene Nm23 Homolog, awd, Regulates Epithelial Integrity during Oogenesis

Abstract

The expression levels of the metastasis suppressor gene Nm23 have been shown to correlate positively or inversely with prognosis in different cancer cohorts. This indicates that Nm23 may be needed at different expression levels and may function differently in various tissues. Here we report a novel epithelial function of the Drosophila melanogaster homolog of human Nm23, abnormal wing discs (awd). We show a dynamic expression pattern of the Awd protein during morphogenesis of the Drosophila follicle cells during oogenesis. Loss-of-function awd mutant cells result in the accumulation and spreading of adherens junction components, such as Drosophila E-cadherin, beta-catenin/Armadillo, and alpha-spectrin, and the disruption of epithelial integrity, including breaking up of the epithelial sheet and piling up of follicle cells. In contrast, overexpression of awd diminishes adherens junction components and induces a mesenchymal-cell-like cell shape change. The gain-of-function phenotype is consistent with a potential oncogenic function of this metastasis suppressor gene. Interestingly, we demonstrate that the epithelial function of awd is mediated by Rab5 and show that the Rab5 expression level is downregulated in awd mutant cells. Therefore, awd modulates the level and localization of adherens junction components via endocytosis. This is the first demonstration of an in vivo function of Nm23 family genes in regulating epithelial morphogenesis.

Progress on Nme (NDP kinase/Nm23/Awd) gene family-related functions derived from animal model systems: studies on development, cardiovascular disease, and cancer metastasis exemplified

“…Early studies in Drosophila showed that awd mutant larval brain exhibited mitotic defects correlated with defective microtubule polymerization (Biggs et al 1990). Subsequently, awd has been implicated in the process of endocytosis in multiple tissues, including neurotransmitter uptake at the neuromuscular junctions (Krishnan et al 2001), surface receptor internalization that modulates the chemotaxis response in tracheal cell and border cell migration (Dammai et al 2003;Nallamothu et al 2008), recycling of adherens junction components (Woolworth et al 2009), and promotion of early-to-late endosome transition that is critical for Notch signaling in follicle cells and in imaginal discs (Ignesti et al 2014). These endocytic functions were associated with the functions of dynamin and Rab5, suggesting a GTP-supplier function for these monomeric GTPases.…”

“…Previous studies have shown that AMPKα mutation could cause epithelial disruption and proliferation in the follicle cells but only under energy stress (Mirouse et al 2007). Since loss of awd results in delocalization of adherens junction and breakdown of epithelial characteristics (Woolworth et al 2009), the AMPK-awd axis may the key component in the regulation of nutrientdependent epithelial integrity.…”

Nucleoside diphosphate kinases (NDPKs) in animal development

“…This expression pattern of AWD is consistent with the work of the Shearn laboratory (discussed earlier), as the ovarian sterile phenotype of awd mutants originates from defects of the follicular epithelial cells. The Hsu laboratory demonstrated that AWD is required to maintain epithelial integrity [55]. Lack of AWD and excess AWD activity both disturb epithelial integrity.…”

The NDPK/NME superfamily: state of the art

“…102,103 Awd-dependent endocytosis also contributes to ensure epithelial integrity of follicular cells in the egg chamber by modulating the levels of adherens junction components. 104 A number of further studies suggest links between NME and dynamin. The endocytic function of VHL (Von Hippel-Lindau) protein, a tumor suppressor that serves as a negative regulator of hypoxia-inducible factor-alpha subunits, regulates surface localization of FGF (fibroblast growth factor) receptor 1 via NME1, 105 and ARF6-GTP, the active form of ARF6, interacts with and recruits NME1 to facilitate dynamin-mediated endocytosis.…”