Journal of Developmental Biology Review Axin Family of Scaffolding Proteins in Development: Lessons from C. elegans Avijit Mallick 1 , Shane K. B. Taylor 1, Ayush Ranawade 2 and Bhagwati P. Gupta 1,* 1 Department of Biology, McMaster University, Hamilton, ON L8S-4K1, Canada; [email protected] (A.M.); [email protected] (S.K.B.T.) 2 Department of Bioengineering, Northeastern University, Boston, MA 02115, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-905-525-9140 (ext. 26451); Fax: +1-905-522-6066 Received: 12 August 2019; Accepted: 11 October 2019; Published: 15 October 2019 Abstract: Scaffold proteins serve important roles in cellular signaling by integrating inputs from multiple signaling molecules to regulate downstream effectors that, in turn, carry out specific biological functions. One such protein, Axin, represents a major evolutionarily conserved scaffold protein in metazoans that participates in the WNT pathway and other pathways to regulate diverse cellular processes. This review summarizes the vast amount of literature on the regulation and functions of the Axin family of genes in eukaryotes, with a specific focus on Caenorhabditis elegans development. By combining early studies with recent findings, the review is aimed to serve as an updated reference for the roles of Axin in C. elegans and other model systems. Keywords: Axin; C. elegans; pry-1; axl-1; WNT signaling; scaffolding protein; signal transduction; development Axin was first discovered as a negative regulator of WNT (wingless and int-1) signaling in mice while deciphering its role in embryonic axis formation [1]. This protein is the product of the mouse Fu (Fused) gene [2–4] and was named Axin for its initial discovered role in inhibiting axis formation (Axis inhibition). Soon after, Axin and its homolog Axil (for Axin-like, also known as Axin2 or conductin) were discovered in a yeast two-hybrid screen for GSK-3β (glycogen synthase kinase-3 beta)-interacting proteins [5,6]. Since then, involvement of Axin family proteins in WNT signaling has been extensively characterized, revealing that they can bind to and facilitate interactions between several WNT pathway components such as the WNT co-receptor LRP (low-density lipoprotein-related protein), Dvl (Dishevelled), APC (tumor suppressor Adenomatous Polyposis Coli), GSK-3β, β-catenin, CKs (casein kinases), and many others [7–18]. Over the years, Axin homologs have been identified in several organisms. Many of these discoveries were facilitated by the availability of whole-genome sequences. While functional studies are currently limited to a few animal models, the findings and sequence data show that Axin is conserved in metazoans, with invertebrates carrying an ancestral gene and higher eukaryotes possessing two distinct Axin1 and Axin2 genes [1,5,19,20]. The nematode lineage contains an additional, divergent, Axin homolog [21] (see Figure1 and further discussion below). Experiments performed in mice have revealed that while both Axin proteins regulate WNT signaling, they have different functions [22–24]. Whereas Axin1 is ubiquitously expressed in embryos and is essential for viability, Axin2 is restricted to a few tissues and serves as a transcriptional target of WNT signaling [25,26]. Similar to the published literature, the ‘Axin1’ and ‘Axin’ names have been used interchangeably in this article, whereas the term ‘Axin family’ refers to both Axin1 and Axin2 homologs that share conserved protein-interaction domains. J. Dev. Biol. 2019, 7, 20; doi:10.3390/jdb7040020 www.mdpi.com/journal/jdb J. Dev. Biol. 2019, 7, 20 2 of 23 J. Dev. Biol. 2019, 7, x FOR PEER REVIEW 2 of 23 FigureFigure 1. AxinAxin family family is is conserved conserved in multicellularmulticellular eukaryotes.eukaryotes. Multiple Multiple sequence sequence alignment alignment dendrogramdendrogram waswas generatedgenerated by by LIRMM LIRMM (http: (http://www.phylogeny.fr/simple_phylogeny.cgi)//www.phylogeny.fr/simple_phylogeny.cgi) using defaultusing defaultprogram program parameters. parameters. 1. Axin Domains 1. Axin Domains Axin possesses multiple regions that facilitate its interactions with various proteins (Figure2). One Axin possesses multiple regions that facilitate its interactions with various proteins (Figure 2). of these regions is involved in regulating G protein signaling (the RGS domain) near the N-terminus One of these regions is involved in regulating G protein signaling (the RGS domain) near the N- that can bind the APC protein [11] (Figure2). In the context of the WNT pathway, APC requires Axin terminus that can bind the APC protein [11] (Figure 2). In the context of the WNT pathway, APC to form a destruction complex with GSK-3β and other proteins [27]. The C-terminus of Axin possess requires Axin to form a destruction complex with GSK-3β and other proteins [27]. The C-terminus of a DIX (Dishevelled/Axin homologous) domain that facilitates WNT pathway-specific interactions Axin possess a DIX (Dishevelled/Axin homologous) domain that facilitates WNT pathway-specific by forming homodimers and heterodimers with the Axin and Dvl proteins [14,28] (Figure2). In interactions by forming homodimers and heterodimers with the Axin and Dvl proteins [14,28] addition to these well-defined domains, Axin also contains regions between the RGS and DIX domains (Figure 2). In addition to these well-defined domains, Axin also contains regions between the RGS that bind β-catenin (in Armadillo repeats 2–7) [5] and two serine/threonine kinases GSK-3β [29] and and DIX domains that bind β-catenin (in Armadillo repeats 2–7) [5] and two serine/threonine kinases CKIα (casein kinase I) [30] (Figure2). Through these regions, Axin recruits APC, GSK-3 β, CKIα, GSK-3β [29] and CKIα (casein kinase I) [30] (Figure 2). Through these regions, Axin recruits APC, and β-catenin to form a multimeric complex in the absence of WNT signaling. The complex causes GSK-3β, CKIα, and β-catenin to form a multimeric complex in the absence of WNT signaling. The enhanced phosphorylation of β-catenin by GSK-3β and CKIα [31,32] and targets it for ubiquitination complex causes enhanced phosphorylation of β-catenin by GSK-3β and CKIα [31,32] and targets it and proteasomal degradation. Activation of WNT signaling inhibits the destruction of β-catenin and for ubiquitination and proteasomal degradation. Activation of WNT signaling inhibits the promotes nuclear translocation of the non-phosphorylated form and activation of WNT-responsive destruction of β-catenin and promotes nuclear translocation of the non-phosphorylated form and genes [32]. Axin also possesses additional sequences that can facilitate protein–protein interactions, activation of WNT-responsive genes [32]. Axin also possesses additional sequences that can facilitate thereby promoting activation of other, non-WNT pathway, factors ([33], also see below). protein–protein interactions, thereby promoting activation of other, non-WNT pathway, factors ([33], also see below). J. Dev. Biol. 2019, 7, 20x FOR PEER REVIEW 33 of of 23 Figure 2. Axin binds to a wide array of proteins. Shown here are the WNT signaling components—APC, Figure 2. Axin binds to a wide array of proteins. Shown here are the WNT signaling components— CKI, GSK-3, β-catenin and Dvl, with known relative binding positions in Mouse Axin1 (NP_001153070.1). APC, CKI, GSK-3, β-catenin and Dvl, with known relative binding positions in Mouse Axin1 The C-terminus domain (DIX) facilitates formation of homo and hetero dimers. (NP_001153070.1). The C-terminus domain (DIX) facilitates formation of homo and hetero dimers. 2. Overview of the Developmental Roles of the Axin Family 2. Overview of the Developmental Roles of the Axin Family 2.1. Vertebrate Models 2.1. Vertebrate Models The Axin family of genes are involved in diverse developmental processes. Analyses of mutant and geneThe knock-down Axin family experiments of genes inare di involvedfferent animal in diverse models developmental have shown defects processes. in anterior–posterior-axis Analyses of mutant formationand gene andknock-down organogenesis. experiments In some in cases,different these animal abnormalities models mayhave also shown contribute defects to in early anterior– stage lethality.posterior-axis As mentioned formation above,and organogenesis. Axin was initially In some identified cases, these for its abnormalities role in embryonic may axisalso formation,contribute asto mutantearly stage mice lethality. exhibited As axial mentioned defects [above,1]. Subsequently, Axin was initially the gene identified was also shownfor its role to be in essential embryonic for viabilityaxis formation, and the as formation mutant mice of manyexhibited other axial organs, defects including [1]. Subsequently, the heart, tail,the gene primitive was also streak, shown brain, to andbe essential muscles for [24 viability,34–36]. and In zebrafish, the formation mutations of many in Axinother causeorgans, abnormal including fate the determination heart, tail, primitive of the eyesstreak, and brain, telencephalon, and muscles with defective[24,34–36]. establishment In zebrafish, of asymmetriesmutations in ofAxin the nervouscause abnormal system [37 ,fate38]. Experimentsdetermination in ofXenopus the eyes laevis andrevealed telencephalon, that a failure with todefective regulate establishment Axin activity results of asymmetries in duplication of the of thenervous dorsal system axis due [37,38]. to the Experiments constitutive in activation Xenopus laevis of WNT revealed signaling that a as failure