Axin is a central component of the canonical Wnt signaling pathway that interacts with the adenomatous polyposis coli protein APC and the kinase GSK3beta to downregulate the effector beta-catenin. In the nematode Caenorhabditis elegans, canonical Wnt signaling is negatively regulated by the highly divergent Axin ortholog PRY-1. Mutation of pry-1 leads to constitutive activation of BAR-1/beta-catenin-dependent Wnt signaling and results in a range of developmental defects. The pry-1 null phenotype is however not fully penetrant, indicating that additional factors may partially compensate for PRY-1 function. Here, we report the cloning and functional analysis of a second Axin-like protein, which we named AXL-1. We show that despite considerable sequence divergence with PRY-1 and other Axin family members, AXL-1 is a functional Axin ortholog. AXL-1 functions redundantly with PRY-1 in negatively regulating BAR-1/beta-catenin signaling in the developing vulva and the Q neuroblast lineage. In addition, AXL-1 functions independently of PRY-1 in negatively regulating canonical Wnt signaling during excretory cell development. In contrast to vertebrate Axin and the related protein Conductin, AXL-1 and PRY-1 are not functionally equivalent. We conclude that Axin function in C. elegans is divided over two different Axin orthologs that have specific functions in negatively regulating canonical Wnt signaling.

Disabled proteins are a conserved family of monomeric adaptor proteins that in mammals are implicated in the endocytosis of lipoprotein receptors. Previous studies have shown that the sole Caenorhabditis elegans Disabled homologue, DAB-1, is involved in the lipoprotein receptor-mediated secretion of a fibroblast growth factor. We show here that DAB-1 is essential for the uptake of yolk protein by developing oocytes, and for the localisation of the yolk receptor RME-2. The localisation of DAB-1 in oocytes is itself dependent upon clathrin and AP2, consistent with DAB-1 acting as a clathrin-associated sorting protein during yolk protein endocytosis. DAB-1 is also required for the endocytosis of molecules from the pseudocoelomic fluid by the macrophage-like coelomocytes, and is broadly expressed in epithelial tissues, consistent with a general role in receptor-mediated endocytosis. We also show that dab-1 mutations are synthetic lethal in combination with loss-of-function mutations affecting the AP-1 and AP-3 complexes, suggesting that the reduced fluid and membrane uptake exhibited by dab-1 mutants sensitises them to defects in other trafficking pathways.

In this review, we look at the processes that occur ‘behind the scenes’ in Wnt signalling, within the Wnt-producing cells. The Wnt community has long been focused upon events that occur downstream of Wnt binding to its receptors, but the recent discovery that the maturation of the Wnt protein may have a profound effect on its signalling properties has excited great interest. In the last 2 years, several key regulators of Wnt production have been discovered, but our global understanding of this process remains relatively poor. Several models that reconcile former and recent observations of Wnt modification, sorting and secretion, and which highlight the potential of this emerging field, are presented here.

Wnt proteins function as morphogens that can form long-range concentration gradients to pattern developing tissues. Here, we show that the retromer, a multiprotein complex involved in intracellular protein trafficking, is required for long-range signaling of the Caenorhabditis elegans Wnt ortholog EGL-20. The retromer functions in EGL-20-producing cells to allow the formation of an EGL-20 gradient along the anteroposterior axis. This function is evolutionarily conserved, because Wnt target gene expression is also impaired in the absence of the retromer complex in vertebrates. These results demonstrate that the ability of Wnt to regulate long-range patterning events is dependent on a critical and conserved function of the retromer complex within Wnt-producing cells.