The Roles of the Wnt-Antagonists Axin and Lrp4 during Embryogenesis of the Red Flour Beetle Tribolium castaneum

Abstract

In both vertebrates and invertebrates, the Wnt-signaling pathway is essential for numerous processes in embryogenesis and during adult life. Wnt activity is fine-tuned at various levels by the interplay of a number of Wnt-agonists (Wnt ligands, Frizzled-receptors, Lrp5/6 coreceptors) and Wnt-antagonists (among them Axin, Secreted frizzled and Lrp4) to define anterior–posterior polarity of the early embryo and specify cell fate in organogenesis. So far, the functional analysis of Wnt-pathway components in insects has concentrated on the roles of Wnt-agonists and on the Wnt-antagonist Axin. We depict here additional features of the Wnt-antagonist Axin in the flour beetle Tribolium castaneum. We show that Tc-axin is dynamically expressed throughout embryogenesis and confirm its essential role in head development. In addition, we describe an as yet undetected, more extreme Tc-axin RNAi-phenotype, the ectopic formation of posterior abdominal segments in reverse polarity and a second hindgut at the anterior. For the first time, we describe here that an lrp4 ortholog is involved in axis formation in an insect. The Tribolium Lrp4 ortholog is ubiquitously expressed throughout embryogenesis. Its downregulation via maternal RNAi results in the reduction of head structures but not in axis polarity reversal. Furthermore, segmentation is impaired and larvae develop with a severe gap-phenotype. We conclude that, as in vertebrates, Tc-lrp4 functions as a Wnt-inhibitor in Tribolium during various stages of embryogenesis. We discuss the role of both components as negative modulators of Wnt signaling in respect to axis formation and segmentation in Tribolium.

Double abdomen in a short-germ insect: Zygotic control of axis formation revealed in the beetleTribolium castaneum

“…S5 B – D and T – V ). Hence, interference with early Wnt signaling alone did not phenocopy the Tc-gcl phenotype in our hands, while recently, double-abdomen phenotypes have been described upon Tc-axin RNAi ( 31 ).…”

β-catenin dependent axial patterning in Cnidaria and Bilateria uses similar regulatory logic

“…into the area where endogenous -catenin signaling intensity is expected to be lower, while upon strong upregulation of -catenin signaling their expression ceases altogether (4). A similar dose-dependent response to "windows" of -catenin signaling intensity was previously demonstrated in axial patterning of deuterostomes (11)(12)(13)(14), and possibly also in protostomes (15,16). This similarity suggests that the regulatory principle behind the "window" behavior may represent the ancestral logic of -catenin dependent axial patterning, however, its mechanism is not clear.…”

A dual role for Axin in establishing the anterior-posterior axis in the early sea urchin embryo

“…The current evidence from vertebrate and invertebrate taxa indicates that cWnt inhibition at the anterior pole of embryos by secreted factors is a relatively late event during embryogenesis. However, recent work in the short germ band insect Tribolium castaneum showed that there is an early inhibition of cWnt signaling at the future anterior end of embryos (Ansari et al, 2018;Fu et al, 2012;Pruhs et al, 2017).…”

“…Interestingly, the inhibition of cWnt signaling at the anterior end of early T. castaneum embryos appears to be mediated by Axin, a critical cytoplasmic component of the βcatenin destruction complex in the cWnt pathway (Ansari et al, 2018;Fu et al, 2012;Pruhs et al, 2017). The β-catenin destruction complex is a highly conserved negative regulator of cWnt signaling, and in addition to Axin, this complex is composed of three other major proteins: APC, the product of the adenomatous polyposis coli gene, Glycogen Synthase Kinase 3β (GSK-3β), and Casein Kinase 1a (CK1a).…”

“…Mutations in Axin and/or APC that lead to impaired βcatenin regulation can lead to increased levels of cWnt signaling (MacDonald et al, 2009;Nusse and Clevers, 2017). In T. castaneum TcAxin is maternally expressed at the anterior end of the unfertilized egg and downregulating Axin expression using RNAi resulted in the duplication of posterior structures at the anterior end of the embryo (Ansari et al, 2018;Fu et al, 2012;Pruhs et al, 2017). These studies have provided evidence that maternal Axin localized to the anterior end of T. castaneum eggs and early embryos downregulates cWnt signaling and blocks the formation of posterior structures at the anterior end.…”