The ADP-ribose polymerase Tankyrase regulates adult intestinal stem cell proliferation during homeostasis in Drosophila

Abstract

Wnt/β-catenin signaling controls intestinal stem cell (ISC) proliferation, and is aberrantly activated in colorectal cancer. Inhibitors of the ADP-ribose polymerase Tankyrase (Tnks) have become lead therapeutic candidates for Wnt-driven cancers, following the recent discovery that Tnks targets Axin, a negative regulator of Wnt signaling, for proteolysis. Initial reports indicated that Tnks is important for Wnt pathway activation in cultured human cell lines. However, the requirement for Tnks in physiological settings has been less clear, as subsequent studies in mice, fish and flies suggested that Tnks was either entirely dispensable for Wntdependent processes in vivo, or alternatively, had tissue-specific roles. Here, using null alleles, we demonstrate that the regulation of Axin by the highly conserved Drosophila Tnks homolog is essential for the control of ISC proliferation. Furthermore, in the adult intestine, where activity of the Wingless pathway is graded and peaks at each compartmental boundary, Tnks is dispensable for signaling in regions where pathway activity is high, but essential where pathway activity is relatively low. Finally, as observed previously for Wingless pathway components, Tnks activity in absorptive enterocytes controls the proliferation of neighboring ISCs non-autonomously by regulating JAK/STAT signaling. These findings reveal the requirement for Tnks in the control of ISC proliferation and suggest an essential role in the amplification of Wnt signaling, with relevance for development, homeostasis and cancer.

Wnt/Wingless Pathway Activation Is Promoted by a Critical Threshold of Axin Maintained by the Tumor Suppressor APC and the ADP-Ribose Polymerase Tankyrase

“…Indeed, small molecule inhibitor studies have indicated that Tnks is important for the Wnt-dependent regeneration of fins following injury in adult fish (Chen et al 2009;Huang et al 2009). Moreover, our recent work has revealed that regulation of Axin by Drosophila Tnks is required for Wingless target gene activation and the Wingless-dependent control of intestinal stem cell proliferation in the adult midgut Wang et al 2016). Importantly, in the midgut, Tnks is essential for target gene activation in regions where the Wingless pathway is activated at relatively low levels, but dispensable at high levels , suggesting a critical role for Tnks in the amplification of signaling.…”

“…Redundancy in Axin degradation pathways would provide a compensatory fail-safe mechanism to prevent an increase in Axin above its threshold and the resultant inhibition of signaling. Functional redundancy in Axin degradation pathways may also explain why no defects in Wnt-dependent embryonic development were observed upon disruption of fish or fly Tnks (Huang et al 2009;Feng et al 2014;Wang et al 2016). Nonetheless, the high degree of sequence conservation present in Tnks homologs suggests that Tnks loss cannot be fully compensated by other pathways in all in vivo contexts.…”

Wnt pathway activation by ADP-ribosylation

“…3, Figs 4f and 5). Furthermore, Drosophila Tnks is essential for Wg target gene activation in the adult intestine, and exclusively within regions of the gradient where Wg is present at relatively low concentration 49,64 . Thus, the context-specific roles of Tnks observed in different model organisms may reflect the mechanisms described herein, which reveal that the Wntinduced association of Axin with LRP6 occurs even in the absence of Axin ADP-ribosylation, but is markedly enhanced in its presence.…”

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

“…The Wingless pathway is also involved in adult tissue self-renewal in Drosophila (Lin et al, 2008). Genetic depletion of proteins in the Wingless pathway, such as Tcf, arr, dsh and pygo, leads to inhibition of Wingless signaling activation which in turn causes over-proliferation of stem cells in the Drosophila midgut (Kramp et al, 2002;Wang et al, 2016a and2016b;Tian et al, 2016). However, inactivation of Wnt signaling in the small intestine of mice decreases the proliferative potential of stem cells (Fevr et al, 2007;Korinek et al, 1998).…”

“…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.…”