The Extracellular Domain of Lrp5/6 Inhibits Noncanonical Wnt Signaling In Vivo

Abstract

Lrp5/6 are crucial coreceptors for Wnt/beta-catenin signaling, a pathway biochemically distinct from noncanonical Wnt signaling pathways. Here, we examined the possible participation of Lrp5/6 in noncanonical Wnt signaling. We found that Lrp6 physically interacts with Wnt5a, but that this does not lead to phosphorylation of Lrp6 or activation of the Wnt/beta-catenin pathway. Overexpression of Lrp6 blocks activation of the Wnt5a downstream target Rac1, and this effect is dependent on intact Lrp6 extracellular domains. These results suggested that the extracellular domain of Lrp6 inhibits noncanonical Wnt signaling in vitro. In vivo, Lrp6-/- mice exhibited exencephaly and a heart phenotype. Surprisingly, these defects were rescued by deletion of Wnt5a, indicating that the phenotypes resulted from noncanonical Wnt gain-of-function. Similarly, Lrp5 and Lrp6 antisense morpholino-treated Xenopus embryos exhibited convergent extension and heart phenotypes that were rescued by knockdown of noncanonical XWnt5a and XWnt11. Thus, we provide evidence that the extracellular domains of Lrp5/6 behave as physiologically relevant inhibitors of noncanonical Wnt signaling during Xenopus and mouse development in vivo.

Wnt/beta-Catenin Signaling and Small Molecule Inhibitors

“…Heterooligomerization of Fz-LRP5/6 is sufficient for activating Wnt/ -catenin signaling as demonstrated elegantly by studies involving chimeric Fz-LRP5/6 and Fz-Dkk proteins [210][211][212]. Evidence suggests that LRP6, in addition to participating positively in Wnt/ -catenin signaling, may also be engaged in an inhibitory role incatenin independent Wnt signaling [213,214].…”

PTK7 modulates Wnt signaling activity via LRP6

“…PCP signaling inhibited canonical activity (Kühl et al, 2001;Yan et al, 2001;Saneyoshi et al, 2002), whereas the canonical Wnt antagonist Dkk1 protein stimulated PCP activity (Caneparo et al, 2007). During both Xenopus and mouse development, high LRP6 protein levels suppress Wnt-PCP activity, whereas LRP6 protein knockdown activates high Wnt-PCP activity, blocking CE movements in explants (Tahinci et al, 2007;Bryja et al, 2009). LRP6 protein seems to balance optimal canonical versus PCP Wnt signaling levels.…”

“…CE in Keller explants and ACs was rescued by excess LRP6 levels, canceling out PCP overstimulation by Wnt11. Thus, LRP6 protein represses Wnt-PCP activity; reciprocally, LRP6 depletion overstimulates Wnt-PCP activity, inhibiting CE (Tahinci et al, 2007;Bryja et al, 2009).…”

“…Thus, early inhibition of canonical Wnt activity prevents later neural CE by eliminating posterior neural cell fates. Also, LRP6 and PTK7 proteins positively modulate canonical activity, while simultaneously inhibiting noncanonical PCP activity to fine-tune optimal Wnt signaling levels (Tahinci et al, 2007;Bryja et al, 2009). These two LRP6/PTK7-requiring functions, neural fate specification and Wnt-PCP inhibition, are not necessarily mutually exclusive.…”

“…LRP6 protein modulates CE via Wnt-PCP activity. In excess, LRP6 inhibits PCP, yet when depleted, LRP6 overstimulates PCP; thus CE movements are inhibited (Tahinci et al, 2007;Bryja et al, 2009). Additionally, LRP6 depletion lowers canonical Wnt signaling in the neural plate, disrupting posterior neural cell fate specification, reducing CE.…”