Wnt function requires the C. elegans AP-2 μ subunit DPY-23. Together with retromer, DPY-23 regulates the intracellular distribution of MIG-14, a Wnt-binding factor required for Wnt secretion. We speculate that newly synthesized EGL-20/Wnt binds to MIG-14 in the Golgi, targeting the Wnt to the cell membrane for secretion. In this model, AP-2-mediated endocytosis and retromer retrieval at the sorting endosome would recycle MIG-14 to the Golgi, where it can bind to EGL-20/Wnt for next cycle of secretion.
Studies in Drosophila
demonstrated a role for endocytosis in the formation of a Wingless gradient. Models based on a nonautonomous requirement for dynamin in Wnt function implicated endocytosis as part of a relay that transferred Wingless from one cell to the next (Bejsovec and Wieschaus, 1995
; Moline et al., 1999
). Other investigators proposed that the Wingless gradient was generated by diffusion. These investigators proposed that the effects of dynamin loss on Wnt function reflected a lack of Wingless secretion from cells expressing the morphogen (Strigini and Cohen, 2000
). While our results do not directly resolve this controversy, the requirement for DPY-23 in MIG-14 endocytosis supports the hypothesis that endocytosis is necessary for Wnt secretion and provides a mechanism for how endocytosis regulates Wnt secretion.
A previous study argued that retromer was not necessary for Wnt secretion, but instead was necessary for production of a functional Wnt (Coudreuse et al., 2006
). These investigators also proposed that retromer was necessary for long-range Wnt signaling, but only played a minor role in short-range signaling. They argued that retromer mutants produced Wnt molecules that could only act on nearby cells but failed to act on more distant cells. Our findings that retromer is required for MIG-14 trafficking and the previous discovery that Wntless, the Drosophila
MIG-14 homolog, is necessary for Wingless secretion are at odds with the interpretation that retromer plays a specific role in production of a Wnt that acts in long-range signaling (Banziger et al., 2006
; Bartscherer et al., 2006
An argument for retromer playing a specific role in long-range signaling was based on the observations that retromer mutants have little effect on processes that require MOM-2 and LIN-44, Wnts that are produced near responding cells (Coudreuse et al., 2006
). Further support for the long-range hypothesis was based on the higher frequency of V5 defects in egl-20
mutants compared to retromer mutants (Coudreuse et al., 2006
). This difference contrasted with the high frequency of QL migration defects in both egl-20
and retromer mutants. The discrepancy between the V5 and QL defects in the two types of mutants was explained by the closer proximity of the V5 cell to the EGL-20 source. The model that retromer plays a specific role in long-range Wnt signaling has led to speculation that the trafficking events regulated by this complex might control the production of a specifically modified form of Wnt (Coudreuse and Korswagen, 2007
; Coudreuse et al., 2006
; Hausmann et al., 2007
), for example, a Wnt that could associate with lipoprotein particles (Panakova et al., 2005
We favor a simpler hypothesis where retromer is required for MIG-14 recycling and where blocked recycling leads to defects in Wnt secretion. Our observation that excess MIG-14 can ameliorate the Wnt phenotypes of dpy-23 and vps-35 mutants is consistent with the notion that low levels of functional Wnts are still secreted in these mutants. We propose that the phenotypic differences observed between retromer and egl-20 mutants may result from differential sensitivities of various responding cells to lowered Wnt levels, and a similar explanation could account for the phenotypic differences between dpy-23 and Wnt mutants.
While the phenotypes of mig-14 mutants have most of the defects displayed by either single Wnt mutants or Wnt mutant combinations, dpy-23 mutants do not exhibit certain Wnt mutant phenotypes. They do not have the severe ALM polarity defects that are exhibited by cwn-1; egl-20 or cwn-1; cwn-2 double mutants and completely lack the PLM polarity defects of lin-44 mutant. Yet the dpy-23 defects in HSN and QL migration are extremely severe. One explanation for these differences between dpy-23 and mig-14 mutants, as well as the differences between retromer and mig-14 mutants, is that different Wnt-producing cells varying in their dependence on AP-2 or retromer to secrete Wnts. We speculate that endocytosis and retromer recycle MIG-14 for multiple rounds of Wnt secretion. If this hypothesis is correct, phenotypic differences could reflect the ability of some cells to synthesize sufficient MIG-14 resulting in less dependence on recycling. Alternatively, independent mechanisms for trafficking MIG-14 could operate in different Wnt-secreting cells.