Gtpbp2 is a positive regulator of Wnt signaling and maintains low levels of the Wnt negative regulator Axin

Abstract

BackgroundCanonical Wnt signals, transduced by stabilized β-catenin, play similar roles across animals in maintaining stem cell pluripotency, regulating cell differentiation, and instructing normal embryonic development. Dysregulated Wnt/β-catenin signaling causes diseases and birth defects, and a variety of regulatory processes control this pathway to ensure its proper function and integration with other signaling systems. We previously identified GTP-binding protein 2 (Gtpbp2) as a novel regulator of BMP signaling, however further exploration revealed that Gtpbp2 can also affect Wnt signaling, which is a novel finding reported here.ResultsKnockdown of Gtpbp2 in Xenopus embryos causes severe axial defects and reduces expression of Spemann-Mangold organizer genes. Gtpbp2 knockdown blocks responses to ectopic Wnt8 ligand, such as organizer gene induction in ectodermal tissue explants and induction of secondary axes in whole embryos. However, organizer gene induction by ectopic Nodal2 is unaffected by Gtpbp2 knockdown. Epistasis tests, conducted by activating Wnt signal transduction at sequential points in the canonical pathway, demonstrate that Gtpbp2 is required downstream of Dishevelled and Gsk3β but upstream of β-catenin, which is similar to the previously reported effects of Axin1 overexpression in Xenopus embryos. Focusing on Axin in Xenopus embryos, we find that knockdown of Gtpbp2 elevates endogenous or exogenous Axin protein levels. Furthermore, Gtpbp2 fusion proteins co-localize with Dishevelled and co-immunoprecipitate with Axin and Gsk3b.ConclusionsWe conclude that Gtpbp2 is required for canonical Wnt/β-catenin signaling in Xenopus embryos. Our data suggest a model in which Gtpbp2 suppresses the accumulation of Axin protein, a rate-limiting component of the β-catenin destruction complex, such that Axin protein levels negatively correlate with Gtpbp2 levels. This model is supported by the similarity of our Gtpbp2-Wnt epistasis results and previously reported effects of Axin overexpression, the physical interactions of Gtpbp2 with Axin, and the correlation between elevated Axin protein levels and lost Wnt responsiveness upon Gtpbp2 knockdown. A wide variety of cancer-causing Wnt pathway mutations require low Axin levels, so development of Gtpbp2 inhibitors may provide a new therapeutic strategy to elevate Axin and suppress aberrant β-catenin signaling in cancer and other Wnt-related diseases.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-016-0138-x) contains supplementary material, which is available to authorized users.

Functions of unconventional mammalian translational GTPases GTPBP1 and GTPBP2

“…GTPBP2 mutations have been linked to neurodegeneration in humans as well (Jaberi et al 2016). However, in Xenopus embryos, GTPBP2 was reported to bind to the transcription modulator SMAD1 and take part in BMP/ SMAD1 signaling (Kirmizitas et al 2014) as well as interact with Axin and act as a positive regulator of Wnt signaling (Gillis et al 2016).…”

A novel GTPBP2 splicing mutation in two siblings affected with microcephaly, generalized muscular atrophy, and hypotrichosis

“…Wnt/β‐catenin signaling is required for polarity of embryos and maintenance of normal tissue homeostasis in Xenopus embryos, and dysregulation of Wnt/β‐catenin signaling causes diseases and birth defects 12 . Reported documents showed that knockdown of GTPBP2 disrupts activation of Wnt signaling and results in axial patterning defects and reduces induction of organizer genes in Xenopus embryos 13 …”

Linkage analysis revealed risk loci on 6p21 and 18p11.2-q11.2 in familial colon and rectal cancer, respectively

“…Moreover, overexpression of PTK7 has been implicated as a biomarker for adenoma and CRC, and is correlated with several clinicopathological features such as TNM stage, tumor differentiation, lymph node and distant metastasis [32, 33]. Similarly, GTPBP 2 is also known as a positive regulator of the Wnt signaling pathway [34], which is involved in tumorigenesis of a wide variety of cancers including CRC. The variant in the gene APCDD1 is also shared among two families.…”