Deletion of the Wnt regulator Znrf3 alters bone geometry without inducing high bone mass.

Abstract

RNF43 and ZNRF3 are transmembrane E3 ubiquitin ligases that negatively regulate Wnt signaling by promoting ubiquitination and degradation of Frizzled receptors. Loss of either gene enhances Wnt/β-catenin signaling and has been linked to tumorigenesis. Wnt signaling is a key regulator of skeletal development and bone homeostasis, and pharmacologic activation of this pathway is an established therapy for osteoporosis. In Xenopus laevis, simultaneous disruption of rnf43 and znrf3 results in supernumerary limb formation; however, their roles in mammalian limb development and skeletal maintenance remain unclear. We demonstrate that mice homozygous for null alleles of both Rnf43 and Znrf3 do not develop supernumerary limbs. Because activation of Wnt/β-catenin signaling in osteoblasts increases bone mass, we hypothesized that osteoblast-specific deletion of Rnf43 and/or Znrf3 would produce a high-bone-mass phenotype. Instead, osteoblast-specific loss of Znrf3 resulted in age- and sex-dependent reductions in trabecular bone mass, characterized by decreased bone mineral density and bone volume fraction, reduced trabecular number, and increased trabecular separation. Cortical bone exhibited increased cross-sectional size with reduced cortical area fraction and altered structural properties, while tissue mineral density was unchanged. In contrast, deletion of Rnf43 had minimal skeletal effects, and combined deletion of both genes did not exacerbate the phenotype observed with loss of Znrf3 alone. These findings support the conclusion that deletion of Znrf3 produces skeletal outcomes distinct from constitutive β-catenin activation and identify ZNRF3 as the predominant RNF43/ZNRF3 family member regulating bone architecture in mature osteoblasts.