Wnt pathway activation by ADP-ribosylation

Abstract

Wnt/b-catenin signalling directs fundamental processes during metazoan development and can be aberrantly activated in cancer. Wnt stimulation induces the recruitment of the scaffold protein Axin from an inhibitory destruction complex to a stimulatory signalosome. Here we analyse the early effects of Wnt on Axin and find that the ADP-ribose polymerase Tankyrase (Tnks)-known to target Axin for proteolysis-regulates Axin's rapid transition following Wnt stimulation. We demonstrate that the pool of ADP-ribosylated Axin, which is degraded under basal conditions, increases immediately following Wnt stimulation in both Drosophila and human cells. ADP-ribosylation of Axin enhances its interaction with the Wnt co-receptor LRP6, an essential step in signalosome assembly. We suggest that in addition to controlling Axin levels, Tnks-dependent ADP-ribosylation promotes the reprogramming of Axin following Wnt stimulation; and propose that Tnks inhibition blocks Wnt signalling not only by increasing destruction complex activity, but also by impeding signalosome assembly.

USP25 regulates Wnt signaling by controlling the stability of tankyrases

“…S5B; Callow et al 2011;Zhang et al 2011;Yang et al 2016). Within 2 h of Wnt3a exposure, the levels of PARsylated Axin1 increased as reported previously (Yang et al 2016), but this increase was notably reduced in Usp25 −/− cells (Fig. 5G).…”

“…It has been reported recently that tankyrase-mediated PARsylation of Axin not only controls its level but also allows PARsylated Axin to directly promote Wnt signaling (Yang et al 2016). In order to detect whether USP25 affects the levels of PARsylated Axin1 in Wnt stimulation, we used a previously developed pull-down assay based on the ability of the Trp-Trp-Glu (WWE) domain of the RING-type E3 ubiquitin ligase RNF146 to bind to PARsylated proteins (Supplemental Fig.…”

“…In order to detect whether USP25 affects the levels of PARsylated Axin1 in Wnt stimulation, we used a previously developed pull-down assay based on the ability of the Trp-Trp-Glu (WWE) domain of the RING-type E3 ubiquitin ligase RNF146 to bind to PARsylated proteins (Supplemental Fig. S5B; Callow et al 2011;Zhang et al 2011;Yang et al 2016). Within 2 h of Wnt3a exposure, the levels of PARsylated Axin1 increased as reported previously (Yang et al 2016), but this increase was notably reduced in Usp25 −/− cells (Fig.…”

Axin proteolysis by Iduna is required for the regulation of stem cell proliferation and intestinal homeostasis in Drosophila

“…Although this may seem somewhat surprising, it is consistent with inactivation of Tnks in Drosophila, which also causes no overt abnormalities (Feng et al, 2014;Wang et al, 2016a and2016b;Yang et al, 2016). Like for Iduna, Tnks mutants have no obvious effects on wing development and the expression of Wingless target genes in larval wing discs, despite the fact that Axin levels are increased (Feng et al, 2014;Wang et al, 2016a and2016b;Yang et al, 2016). Our interpretation of these findings is that most tissues can tolerate relative modest (2-3fold) changes of Axin.…”

“…The principal components of this complex are adenomatous polyposis coli (APC), mice are overall normal; however, double knock-out of Tnks1/2 causes early embryonic lethality, which indicates their redundancy in mouse development (Hsiao et al, 2006;Chiang et al, 2008). On the other hand, inactivation of the single Drosophila Tnks gene produces viable flies that have slightly increased Axin levels but no overt defects (Wang et al, 2016a and2016b;Feng et al, 2014;Yang et al, 2016;Tian et al, 2016). Therefore, the exact physiological function of TNKS and Iduna-mediated regulation of Wnt-signaling remains to be determined.…”